BB King, who has just turned 87, has returned home to Mississippi to play to family and friends. In the experience of a lifetime, Ed Vulliamy joins him and hears from the maestro about his rise from the cotton fields to international stardom

The fat red sun settles itself against the horizon, throwing a last, honey-sweet light through humid evening and over a small crowd on the lawn beside a railroad track that cuts through the cotton fields beyond. A quarter-moon rises and a chorus of cicadas serenades imminent twilight, now conjoined by the sound of the band; the drummer catches the backbeat and the compere announces: "How about an Indianola hometown welcome for the one-and-only King of the Blues: BB KING!"

And on he comes, to applause from people who know him well and claim him as their own – the last of the blues masters a few weeks short of his 87th birthday. "Nice evening, isn't it?" he says, and introduces his nephew on sax. Some of his 15 children (all by different mothers) and innumerable grandchildren are in the audience, though one of his daughters died recently of diabetes, as had BB's mother – a poignant riptide beneath the occasion. "I guess you can look at me," he says from the stage, "and tell I'm the old man. My name is BB King."

Backed now by a lilac glow in the western sky – and looking east towards the village of Itta Bena, where he was born – BB sits down and starts up the show. He reaches "Key to the Highway", and there it is: that one long and trembling note, hanging there in the wafts of barbecue smoke, like only BB King can play it. He rolls his eyes, raises his eyebrows, then stares out into the crowd – and there's a collective gasp, a ripple of applause, and a mutual bond of affection.

This is a huddle, not a crowd, really. The town has come to hear its famous son: mostly black people – in families, many with a picnic – plus a few whites with ponytails, ZZ Top beards or other gestures of nonconformity. There are people here like Alfred Knox – one of 11 children with eight of his own (and 21 grandchildren) – who left Mississippi for Milwaukee when he was 19, the sound of Honeyboy Edwards playing juke joints ringing in his ears, and has now come back with his nephew Gervis to hear BB, to hear and talk blues, talk politics. The usual jocks and suits who wave bottles of Bud and shout at tourist clubs like BB King's own franchise in Memphis are not here for this annual homecoming concert – oddly, but thank God.

Nor, indeed, are some of Indianola's good citizens. Latunya and her friend were in the post office earlier, and said how "We're real excited BB's coming back. Gee, I'd lo-o-ove to go see him play. But I go out Fridays. I don't go out Wednesdays, I only go out Fridays". This is also the town in which the White Citizens Council was formed, political wing of the Ku Klux Klan; and the founders' heirs are probably elsewhere tonight.

The maestro's sonority on guitar is as inimitably perfect as ever. After one long, searing note during "The Thrill is Gone", BB King darts the stare of a clown right into the front rows, as though to say: "How about that!?" But it is BB's voice on the warm breeze that stops a heartbeat – that feeling behind and between the words that is the quintessence of the blues.

This is the 35th Homecoming concert, an event initially staged in memory of Medgar Evers, the civil rights activist and friend of BB's assassinated by a member of the White Citizens' Council. There is something important about BB King's appearance, for the simple reason that even if he is still touring as a 90-something, there will not be that many more of these; nor will there be occasions upon which this great man will reflect on his extraordinary life, which began when he was working out there on the cotton plantations, living alone in a shack, having outlived his mother and then his grandmother.

All the more reason for that remarkable life to be recounted on film. Jon Brewer's The Life of Riley, released next month, was made after hours talking with BB King, with those who grew up around him, and those – including many British musicians – who followed his example. This is the definitive filmed testimony of the last great bluesman.

Even more important than the tributes from stars – Eric Clapton, Keith Richards, Bono et al – are the recollections of people like John Fair and Clemmie Trevellaine, who sit on the porch of their home remembering the day that nine-year-old Riley B King cycled home to live among the cotton fields, working behind a plough, holding the reins of a mule. "I knew none of this," says Brewer. "I had no idea this had been BB King's life. That he lived alone, in a shack out there, at nine years of age, working for four years to pay off his dead mother's and grandmother's debts. I never knew he was so alone, talking to rabbits, his only friends. And by the time he's telling me this, I'm in tears."

So it's also a good time for the Observer to speak directly to the master in his hometown, just after his bus pulled up in BB King Street, where, before performing, he broke ground for a new pavement complete with flagstones which will tell his life story. The story he now talks about as we sit in the shade of a pecan tree: : "I don't do this," he smiles, "but I heard you come all the way from England". "Got here a short while ago", I replied, "two hours sleep – and I never thought I'd see the day, when I was 16 and bought this" – it's a copy of his album, Indianola Mississippi Seeds. BB laughs and signs the record; his affection for England is musical, not sentimental.

There's a turn off the main road into town that leads through the low-slung fields to Itta Bena. The season is waning now: just north of here around Clarksdale, capital of the blues, little clouds of cotton have burst forth "like popcorn", as John Steinbeck once remarked; here the white puffs remain closed in their pods. When he was a boy, BB King used to drive his mule through those fields while his Uncle Jack, up front, sung "the holler", the descendant of the slave chants and responses, wherein the blues began.

"I remember the holler," says BB. "Holding the reins of a mule pulling a hoe through them cotton fields." The field holler, he explains, was a lament sung on a minor scale by a single voice. It also functioned as a communication to alert others in the field that the boss was coming, or that water was needed. "Yeah, the holler is where it all started. I think it's in all of us."

Riley B King was born on 16 September 1925, great-grandson of a slave. He moved soon after with his mother to Kilmichael, in Hill Country, to work as a farmhand and listen to his aunt's Blind Lemon Jefferson records. His uncle, the great bluesman Bukka White, would visit from Memphis – play and sing.

First BB's mother died, then the grandmother in whose care he had been placed. "The blues," he would later write, "was bleeding the same blood as me." Picking cotton for 75 cents a day, BB had coveted the local preacher's guitar and was even allowed to "play a few chords he taught me". He then made his own, he says: "We used to make it with wire, the kind you use to make a cotton bail, and tie it to a broom handle. Clamp it down, the sound changed, and I'm playin' music." Later, his employer, a white man called Flake Cartledge, advanced him the $15 he needed for a real one.

There followed an unhappy period with his father's family way south in Lexington. There, BB witnessed the lynching and castration of a young black man by a white mob – his crime had been to wolf-whistle at a white girl. Riley King fled Lexington, and his father's new family, cycling alone "like a bat out of hell, never to return", back to Itta Bena.

Nine-year-old Riley King was now back in the Delta, where he worked the fields. picking "Delta Pinelet"."Cotton was a force of nature. There's a poetry to it, hoeing and growing cotton," he later wrote. Under the pecan tree, he adds: "Though it didn't make none of my people rich. I figured out that I must have walked around the world, all those hours and days and weeks behind a mule." His biographer Charles Sawyer once asked King: "How can you play in 90-degree heat in a 3-piece suit?" To which BB replied: "I used to work all day in the hot sun in Mississippi." Sawyer shook his head and said he would pass out if I tried to play under all those layers of fabric. BB replied: "Well, Charlie, you're white".

On Saturday nights, workers left the fields for town. "I was peeping through the slats at a place I'm playing later tonight they call Club Ebony," recalls BB, "to hear Count Basie play, Charlie Parker, too – and see all those beautiful women in tight dresses jivin' away." Young Riley King sang gospel on the street corners of this town, and learned soon that "the church folks that liked my singing didn't slip me a dime much as the other folks did when I changed sidewalks to the other side of town and changed praising the Lord to praising a lady", which is another cornerstone of the blues King learned early: it is not just the words of a song, it the mood that matters, convoyed by the larynx, as instrument. That is what defined and defines BB's extraordinary voice. Then there is the instrument itself, the guitar.

Several adventures later, BB fulfilled his dream of reaching Memphis to play. He recalls a crucial moment in the history of the guitar style which became his and his alone – the vibrato that is instantly recognisable as BB King's after only a single note. He was staying with Bukka White, and "Bukka used to play slide using a bottleneck, or just a piece of pipe. I wanted to do that, and I tried and he showed me how – but I got stupid fingers, see, and I just couldn't do it." However: "The sound Bukka made went all through me, and I devised my own technique for producing the tremolo without the slide." He calls it "the butterfly. I swivel my wrist from my elbow, back and forth, and this stretches the string, raising and lowering the pitch of the note rhythmically. With my other fingers stretched out, my whole hand makes a fluttering gesture, a bit like a butterfly flapping its wings."

Thus the BB King sound was born – a sound that expresses mood, feeling, blue or otherwise. In Brewer's film, there is a montage in which most of the great guitarists of their generation acknowledge it, and the ability to recognise it. King himself says it another way: "I tried to connect my singing voice to my guitar an' my guitar to my singing voice. Like the two was talking to one another." Jon Brewer confirms a detail which explains the dialogue between voice and guitar differently: "BB's secret is that he can't sing and play at the same time."

BB King called his guitar Lucille after someone else's fight in a juke joint knocked over a container of kerosene that was the heating system. The place caught fire, the crowd fled, but BB realised he had left his guitar inside and ran to get it. Chastised for his recklessness, he discovered that the fight had been over a girl called Lucille. "I named my guitar Lucille," he says in The Life of Riley, "to remind myself not to do something like that again, and I haven't."

The blues went north, with the great migration from the mechanised cotton fields, in pursuit of work in the big cities. For the most part they accompanied the majority of the black exodus to the burgeoning megalopolis of Chicago. Each musician from what became the golden age of the blues mutated the Delta sound to form their own style – Elmore James, Muddy Waters, Howlin' Wolf – within that Windy City electric sound. But BB King chose a different scene, on Beale Street in Memphis, closer to home up the Mississippi reaches. He came to forge a more rounded, less feral timbre than Muddy or the Wolf, with a bigger band including brass and rhythm sections.

Blues Boy King – shortened to BB – became a radio star at WDIA, the blues station in Memphis. He built his band, he ensured that he tour on a bus, he made lifelong friends, he left his first wife and indulged his love of women – and released his first hit, "Three O'Clock Blues".

In The Life of Riley, Calvin Owens – trumpeter in the original BB King band – recalls those days on what was called the segregated Chitlin' Circuit, named after animal offal that black people were said to enjoy eating, but not whites – "Though I never called it any Chitlin' Circuit," says BB King. "The road," says Owens, "is home", and it remained BB's home for the rest of his life. Sometimes he did 350 performances a year – "I like my job," he says, under the pecan tree. BB King married a second time, to Sue Carol Hill, daughter of the owner of the Club Ebony – founded in 1907 – where BB used to peep and later play. Their marriage, while it lasted, was lived on the road. Staying, of course, in segregated black hotels, eating at segregated black restaurants.

"I've put up with more humiliation than I care to remember," King wrote in his autobiography Blues All Around Me. "Touring a segregated America – forever being stopped and harassed by white cops hurt you most cos you don't realise the damage. You hold it in. You feel empty, like someone reached in and pulled out your guts. You feel hurt and dirty, less than a person." There was one night at the Gaston Hotel in Birmingham, Alabama, where BB King was staying at the same time as Dr Martin Luther King, when "they bombed the place. The bomb rocked my room." Now BB King is back to be honoured in the city where the White Citizens' Council was formed.

Last time I was in this area, around Clarksdale, it was to report on poverty – and in Mississippi, that means black poverty. It was the year of the election between Al Gore and George W Bush, and I sat with old Ruby Walker on her porch with most of her 22 cats frolicking around her. The next few days would be particularly hard for the Walker family, since they were to have marked the 19th birthday of Ruby's granddaughter, Sandra Handy. However, there was a different anniversary: "She was just crossing the railroad line on her way home this time last year when the shooting started and a bullet hit her right in the head." There is a macabre intimacy to the murder. Ruby gestured towards the house behind her, and folks hanging out their washing to dry in the muggy heat: "It was their boy done the shooting. He only got 10 years and it's hard to look at them every day." He was, she adds, "one of the gangbangers – fighting for territory".

Like BB King's, Ruby's great-grandparents were slaves working these cotton fields. "And so was I, after a fashion," she reflected. "We worked from sun-up to sundown, and the money was cheap." Her daughter Mary became the fifth cotton-picking generation until she found work in a local school. But, added Ruby, the furrows deepening across her brow: "I sometimes wonder if they ever really did do away with slavery. I don't know what's happening round here no more. All I know is it was better in the 40s than it is now."

That same visit I met a relative of the Fair family, who shared a plot with young BB King, and who – in Brewer's film – recalls him as a boy cycling home from Lexington. Shirley Fair was the owner of a flower store in Clarksdale called Ooh So Pretty Flowers, where a meeting was arranged with President Bill Clinton when he blew through on his Poverty Tour of 1999. But, said Mrs Fair, "nothing has gotten any better. There's nothing here to grasp on to. The railroad is closed, the factories have gone. The good folks move on and the gangbangers take over the streets, and that makes it harder for businesses to survive. The president wrote me a letter saying I could apply for a grant and employ 10 people. But there were so many rules and regulations I couldn't understand nothing. Always something to slow you and stop you. So I did stop."

Nights round here are different when BB King is not in town. Down at the Club Sugar in Jonestown – a ramshackle brick cabin beside the railroad track – lads like Q, with his blue bandana, are arriving to drown their sorrows and thoughts in the barrage of rap music while his friend Icy Man, although in the middle of a conversation, pulls hard on a tube of "rocks", his eyes glazing over, his face frozen in oblivion. It's like finding the old 70s Bronx in the middle of a 21st-century cotton plantation, and the crowd soon moves to a joint nearer the village centre, Hot Spot. We ask if we can take pictures of the heaving and dancing, to be told politely that this would be a "bad idea because a lot of the guys here are wanted. They're serious." Our host, a man called Clee, who dislikes rap and who dreams mainly of setting up a blues band in town, adds: "They're not local, they're County."

BB King knows all this. And he is not as self-satisfied as one might expect him to be with "progress" since the bad old days. For sure he jammed at the White House with a black president, and there's a BB King Day in the Mississippi calendar – but also the confederate ensign is its state flag. "We've come a long, long way," says BB King, "but we ain't come far enough. There's still a long, long way to go." I ask if he is aware of his own role in that "long, long way" since he saw a man lynched and shared a bombed-out motel with Martin Luther King. "I'd like to think I made a lil' footprint in the sand," he replies, and puts his arm around a young boy next to us, on his right. "This here little boy's the same age as me when I was holding the reins of a mule. He won't never know those times, but I wonder what this boy will grow up to do. I wonder…"

However: "The racists couldn't legislate musical taste," writes BB. The influence of BB King and the blues over great white musicians from the generation that followed him is a slice from one of the most compelling stories in 20th-century music and culture: how white hipsters, many of them British, seized on the Delta and Chicago blues, bringing those who sang them before a vast white – and ultimately global – audience, to the amazement of the elderly bluesmen themselves.

There is a wonderful moment in The Life of Riley concerning a gig BB King was playing in Chicago at which four white people arrived. "One of them was extra white," recalls BB, referring to the albino Johnny Winter, a master blues guitarist with long straight hair and pencil-thin tattooed arms. BB was afraid they were from the IRS tax authorities, a perennial bane of his life. Winter asked if he could "sit in on a number", and the master was clearly ambivalent. But: "He was good. I tell you, he was good."

By the time BB King recorded perhaps his greatest album, Live at the Regal, in 1964, mainstream America and much of black America had forgotten about the blues in favour of rock'n'roll and embryonic soul. But the hipsters remained true. John Mayall nurtured a stable that included Eric Clapton, Peter Green and Mick Taylor later of Cream, Fleetwood Mac and the Rolling Stones – and lesser-known names propelled the British blues explosion: Chicken Shack, Savoy Brown, Wishbone Ash, Keef Hartley …

Jon Brewer recalls the moment: "You had these new bands playing bingo halls and Meccas, supporting Cliff Richard or Engelbert Humperdink or whoever, and wondering: 'Which way do we go?' And they found this really good stuff played by black musicians that was far superior to what we had here. So you had people like John Mayall and Peter Green thinking: 'This is the real thing.'"

(My own interest was deep and irrevocable from the night that Son House, Bukka White, Sonny Terry and Brownie McGhee arrived to play at Hammersmith Odeon in 1967. I was compelled, edge-of-seat, and hooked for life. When the white bluesmen went to America, recalls Keith Richards in Brewer's film: "They wanted to know what we were doing and why we were doing it.")

"I can tell BB from one note," says Eric Clapton, in Brewer's film. "Most of us can." He has, says Clapton, "a certain melodic sense unique to himself."

Bill Wyman recalls BB King's tremolo when the two came to record together in 1971: "He showed me how to do it – buggered if I could."

There is an extraordinary sequence in the film where BB describes his arrival at Bill Graham's Fillmore West in San Francisco, when queues of white hippies were in the street. At first BB thinks he is in the wrong place but confesses that he was so moved by several standing ovations he "cried back up the stairway". In our interview under the pecan tree, King calls it "my breakthrough moment. It was an unusual situation. You had all these people playing the blues: Mike Bloomfield and Elvin Bishop were good, Clapton was good, Johnny Winter was good, Peter Green was good." And he leans forward as if to impart some secret: "They had something. They introduced us to a whole new world. And we learned a lot from people like Clapton and Peter Green. I ain't gonna tell you what it is," he says in a whisper, "but we learned something. They did something for the blues."

BB King once wrote, when discussing his technique, that "Lucille was singing the blues better than me." But now he says something intriguingly different: "You've heard me call myself a bluesman and a blues singer," he says. "I call myself a blues singer, but you ain't never heard me call myself a blues guitar man. Well, that's because there's been so many can do it better'n I can, play the blues better'n me. I think a lot of them have told me things, taught me things. But they just ain't me, that's all. They're not BB King." It's a remark that emphasises the primacy of BB King's voice, that all-expressive rasp, that howl of desire or of pain.

I've had BB King to myself for 25 minutes now, surrounded by people who've known him for decades. Old friends and family plus young fans come to behold him for the first time, and it feels like a levee is about to break – not to unleash the Mississippi backwaters, but this throng of good people.

BB King ends our interview with an observation that hallmarks true greatness. In the film he had said: "When I hear what I want to hear, I'd have to stop." Now he elaborates: "I think I've done the best I could have done. But I keep wanting to play better, go further. There are so many sounds I still want to make, so many things I haven't yet done. When I was younger I thought maybe I'd reached that peak. But I'm 86 now, and if I make it through to next month, I'll be 87. And now I know it can never be perfect, it can never be exactly what it should be, so you got to keep going further, getting better."

On the subject of women, BB King's book is markedly – almost disarmingly – candid. And he says now: "I never met a woman I didn't like. I love 'em all, in their different ways" – and sure enough, just then a lady demands his attention.

"You know," he muses in my direction, "if I find myself a nice wife who'll give me a chance, I may just come back to Indianola!"

He turns to the lady in her splendidly coloured dress: "Are you married?

"No sir," she replies.

"You shouldn't be telling me that kinda thing," confides BB.

When BB King leaves the stage in the park next to his museum in Indianola, the night is just beginning. Not only for this part of town, on very much the wrong side of those railway tracks – where blues juke joints and rural slums are huddled against one another and hopping to life – but for the 86-year-old also.

Two hours after bidding farewell, BB is due to take the stage again at what is one of the most historically charged venues in America, that same Club Ebony, founded in 1907, where as a boy Riley B King would peep through the slats wide-eyed at the "jitterbugging, snake-hipping and trucking", as he describes it.

Around 11pm BB King appears onstage, much of his audience one over the eight, talkative but mellow, ready for what is (in my book at least) the experience of a lifetime. The blues master who has played to stadia and venues across the world settles on his chair as if in his own living room, which in a way this is. There's a power problem: "Guess I didn't pay my bill on time," he chuckles.

Then he picks up Lucille and plays those notes – long notes, impossibly stretched, at moments crashing into some zone Hendrix might have navigated. Perhaps oddly, King indulges in less eye contact with the juke-joint tables across which the beers and liquor flow than he did his outdoor audience. He greets them, and banters of course: "Well, sweet ol' Indianola!" – but is in a world of his own now, less the showman than the musician listening hard to his own alchemy.

Outside in the sauna of night, poor young men in gangland attire gather to watch those with tickets a-coming and going; there's a buzz around the big event, never mind at it – drugs for sale, police bundling someone into a van, guards patrolling the visitors' parked cars and lads eyeing them up.

BB swings through his all-time greats now. The air fills with whisky fumes and a surprising level of chatter, and there are curses exchanged with a redneck family, one of whom is blind drunk, unsteady on his feet, blocking people's views and trying to steal an elderly man's fedora hat. For "Every Day I Have The Blues", BB's voice is like that instrument again, at once guttural but velvet, while he apparently sings with his guitar, Lucille talking back to the singer. There's a rendition of "See That My Grave Is Kept Clean" – that great blues anthem about the imminence of death that makes the blood run cold: it has a chunky-rolling feel to it, pierced by the searing defiance of King's guitar, and the nearest he gets to a field holler tonight even if it's on Lucille. "Not bad for an 86-year-old," he says, signing off the night that has now become early morning in cotton country.

The Life of Riley is out on 15 October