President Obama is now comfortably into his crucial first 100 days, and perhaps just at this moment, before the arrival of those Macmillan-esque "events" which could cloud or modify our perception of him, there is little left to say about Obama the pioneer, Obama the politician, Obama the mould-breaker or Obama the icon. But maybe there is something left to notice about Obama the film critic.

In his autobiography, Dreams from My Father, Obama recounts his spell in New York in his youth, studying at Columbia University, from where he graduated in 1983. In his first summer in New York, Obama is visited by his sister Maya and his mother, Ann – famously the woman from whom the president gets the white side of his mixed-race ancestry. (She would die of cancer in 1995 at the age of 52; his father, the Kenyan governmental economist Barack Obama Sr – whom he hardly knew – died in a car crash in 1982 at the age of 46.)

Obama wryly describes his mother and sister almost immediately fussing about the studenty squalor in which he was living: "'He's so skinny,' Maya said to my mother. 'He has only two towels!' my mother shouted as she inspected the bathrooms. 'And two plates!' They both began to giggle."

Maya and Ann cheerfully spend their days doing tourist stuff and get lectured by the stern Barack in the evening on how frivolous they are. Obama writes:

"One evening, while thumbing through the Village Voice, my mother's eyes lit on an advertisement for a movie, Black Orpheus, that was showing downtown. My mother insisted we go see it that night; she said it was the first foreign film she had ever seen."

He goes on:

"'I was only sixteen then,' she told us as we entered the elevator. 'I'd just been accepted to the University of Chicago – Gramps hadn't yet told me I couldn't go – and I was there for the summer, working as an au pair. It was the first time I'd ever been really on my own. Gosh, I felt like such an adult. And when I saw this film, I thought it was the most beautiful thing I had ever seen.'"

Black Orpheus is the 1959 film by Marcel Camus, recreating the Orpheus and Eurydice myth in the Rio carnival; it won the Palme d'Or at Cannes that year and also a Golden Globe and an Oscar for best foreign-langauge film a year later. I wrote a very short review of it when it was revived here in the UK in 2005 and I praised it for what I found to be its innocent charm, rather than the throbbing samba-style vitality which was found to be so compelling at its release.

But for the young Barack Obama, neither aspect was persuasive. He recalls:

"We took a cab to the revival theatre where the movie was playing. The film, a groundbreaker of sorts due to its mostly black, Brazilian cast, had been made in the fifties. The storyline was simple: the myth of the ill-fated lovers Orpheus and Eurydice set in the favelas of Rio during carnival, in Technicolor splendour, set against scenic green hills, the black and brown Brazilians sang and danced and strummed guitars like carefree birds in colourful plumage. About halfway through the movie I decided I'd seen enough, and turned to my mother to see if she might be ready to go. But her face, lit by the blue glow of the screen, was set in a wistful gaze. At that moment I felt as if I were being given a window into her heart, the unreflective heart of her youth. I suddenly realised that the depiction of the childlike blacks I was now seeing on the screen, the reverse image of Conrad's dark savages, was what my mother had carried with her to Hawaii all those years before, a reflection of the simple fantasies that had been forbidden to a white, middle-class girl from Kansas, the promise of another life: warm, sensual, exotic, different."

And this movie, and his mother's undiminished rapture at it, was to be the subject of fierce self-questioning about his relationship with her: "The emotions between the races could never be pure; even love was tarnished by the desire to find in the other some element that was missing in ourselves."

For what it's worth, I think Obama is wrong about Black Orpheus – he's too tough on it. And yet for me this passage exposed, more dramatically than anything has in a very long while, the fact that critical perceptions are governed by class, by background and by race. I saw Black Orpheus as a white man, a white liberal. Of course I did. The assumption of progressive good faith on race, and the indulgence of potential condescension or even stereotyping in an old movie is something that a white liberal can afford, and as far as the arts and culture are concerned in the prosperous west, white liberals are in the ascendant. But Barack Obama responded to the film quite differently. He responded with impatience, with scepticism and with pain; he saw no reason for black men and women to be objectified – and now, as the president of the United States, he is the subject, the most important subject in the world.

Before Barack Obama's presidency, Black Orpheus was perhaps destined to be something for film buffs only. Now, rightly or wrongly, it may become a classic text, a text about something quite other than that intended by its director, Marcel Camus: a loss of liberal innocence about racial difference.