The last Jerry Sadowitz show I saw, one year ago, began with the notorious "magician, comedian, psychopath" (his words) galloping on stage in full Jimmy Savile regalia – blond wig, tinkling jewellery – and crowing about his infant sexual conquests. Not the stuff of blithe good cheer, then – but not cheap either, given that Sadowitz had first branded Savile a paedophile 25 years before the news went public.

Either way, that overture looks positively jaunty next to this year's opener, a video appeal from the mother of the late actor Mark Blanco, who died aged 30 in 2006 after falling from the balcony of an east London flat where rock star Pete Doherty and friends were partying. Police swiftly concluded there were no suspicious circumstances. Sadowitz and Blanco became friends after meeting at a magic convention. The comedian has helped Blanco's mother Sheila fight her seven-year campaign for a full investigation into her son's death. "Funds are desperately needed to expose this miscarriage of justice," Sheila tells Sadowitz's audience.

No matter their commitment to the cause, not many comics would start their show like this. But the usual rules don't apply to Sadowitz. For a start, his audiences are used to laughs sitting very uncomfortably next to brutality and tragedy. A legend in standup, Sadowitz is its original nothing-is-sacred, troubled genius, an unclubbable talent shunned by telly while his imitators – Frankie Boyle, Jimmy Carr, Ricky Gervais – made their millions.

But Mark Blanco isn't – or at least, isn't yet – part of Sadowitz's act. "It's good that the audience sees that message from Sheila as a separate thing," he tells me. "I've lost one of my best friends," he said of Mark in an earlier interview, "and this country has lost a potentially great author, wit and unique talent." Now, he says, "It's pretty disturbing that there was no investigation. It was swept under the carpet."

Sheila and Mark Blanco. Photograph: Stuart Clarke/Rex

Sadowitz makes no bones about the fact that he thinks there was foul play on the night of 3 December 2006. We know that Blanco went to the party to invite Doherty to attend a play he was starring in – Dario Fo's Accidental Death of an Anarchist, about a man who falls, or is thrown, from the fourth floor window of a Milan police station. We know that Blanco was punched, had his hat set on fire and was ejected from the party. CCTV footage shows him returning, and 56 seconds later, falling from the balcony. It then shows Doherty and friends running past Blanco's inert body.

Shortly afterwards, Doherty's minder Johnny "Headlock" Jeannevol confessed to police that he had pushed Blanco over the balcony. He later retracted the statement, while police concluded his death was suicide, or an accident. "Everything surrounding that flat and the people in it," says Sadowitz now, "seems well protected."

Standup is the most immediate art form for speakers of truth to power – where the unsayable can be said first. Sadowitz was, after all, naming Savile's crimes decades before anyone else wanted to listen. So might he consider bringing his opinions on the Blanco case – and what he sees as a cover-up – more directly into his act? "I may do," he says. "I'd be worried that an audience might think I was not serious about it." I'm not convinced by that objection: the line between serious and comical in Sadowitz's world is vanishingly thin. "If I was doing pure standup rather than a magic show, then yes the subject might come into it," he concludes, "and it may well do in the future."

In December 2012, Newsnight commissioned analysis of CCTV footage of the fatal fall, revealing another figure on the balcony and concluding that Blanco was "probably dropped". Police have recently undertaken to review that new evidence. But "it's not in my nature to be optimistic," says Sadowitz. "You know, society always seems to wait for 30 to 50 years and then say, 'Oh, why didn't we listen to somebody all those years ago?' I'm quite defeatist." Not so defeatist, though, that he would stop screening Sheila Blanco's appeal, nightly, in a West End theatre. "You have to at least try. You can't let the details disappear. This was a really decent guy, a good guy. What Mark's mum is doing is unbelievable: she's got a mountain to climb, she has to discuss this with the press, the legal team, everyone, without cracking up. It's very, very tough, and I'll help her if I can."

• For more information on Justice for Mark Blanco, visit justiceformark.com