While the staff at Buckingham Palace were completing their preparations for a visit from the most powerful man in the world, the following exchange took place at the Grosvenor House Hotel:

"You're a nigger's bitch."

"Excuse me?"

"Yeah, nigger?"

"What did you say?"

"Nigger."

"Would you mind not using that word, please?"

"What, nigger? Nigger's not offensive. Nothing wrong with nigger. I know loads of niggers."

While the president of the world's only superpower was putting the final touches to the speech he planned to give to both houses of parliament, an ad appeared in a newspaper, and on billboards around the country. The ad had a purple background, and, in the middle of it, lying in a pool of crystals, or something that an advertising "creative" would probably call "bling", a bar of Cadbury's Dairy Milk Bliss. Above it, in white letters, were the words: "Move over, Naomi, there's a new diva in town."

Naomi Campbell is considering legal action."It's upsetting," she said, "to be described as chocolate, not just for me, but for all black women and black people. I do not," she said, "find any humour in this. It is," she said, "insulting and hurtful."

Kraft Foods, which bought Cadbury in 2010, and broke its pledge to keep its Keynsham Plant open, and whose chief executive has twice refused to appear before a select committee, issued a statement. It was, it said, "never our intention to cause any offence". The campaign, it said, "is a light-hearted take on the social pretensions of Cadbury Dairy Milk Bliss".

James Brown, the celebrity hairdresser who initiated the "nigger's bitch" conversation at the Baftas last week, has also issued a statement. "Everyone who knows me," he said, "knows I am not racist in any way whatsoever". The problem, he said, was not to do with racism. The problem, he said, was drink. He was, he said, "very sorry and very embarrassed".

At the time, what he said to Ben Douglas, who runs children's theatre schools, and lives in Surrey, was this: "I've lived in New York for years," he said. "I know loads of brothers," he said. "Don't take this the wrong way," he said, "but some of my cousins have been with blacks."

Ben Douglas, who wrote about the incident in the Mail on Sunday, but didn't name Brown (who has now named himself), didn't ask him about his family arrangements. He didn't ask him if he knew "loads of brothers" because he was a Catholic, and his parents were big fans of Vatican II. He didn't ask him why he should be interested in the fact that someone he didn't know knew his own brothers, or if, perhaps, he was using the word "brothers" to mean "black men", in which case they weren't his brothers. He didn't ask him why he should be interested in his cousins' sexual partners. He didn't ask him any of this because he was, he said, "speechless". He felt, he said, "annihilated".

Naomi Campbell's mother, Valerie Morris, didn't ask how it was possible for a Cadbury Dairy Milk Bliss to have "social pretensions" since it was, you know, a bar of chocolate. She didn't ask if perhaps the ad had been dreamt up by Berlusconi. She just said that she was "deeply upset" by it. "Do these people," she said, "think they can insult black people and we just take it?"

The answer to her question is yes. But "these people" don't think they're insulting anybody. They think it's fine to compare a black person with a chocolate bar, which is what quite a few black children get called by their white classmates at school. They think it's fine to refer to a black person's "sun tan". They think it's fine to say pretty much anything, as long as it's "light-hearted", which I think means that the person saying it thought it was funny.

Some of them even seem to think it's fine for a white person to call a black person "nigger". They think it's fine for a white person to call a black person a "nigger" because they've heard a black person, perhaps in a TV series, perhaps on an album, perhaps on a street corner, from someone who wants to be in a TV series, or making an album, calling another black person a "nigger". They think that because a group of people in one particular subculture have decided, for complicated reasons, to reclaim the worst word of abuse that can be directed at a black person, it's fine for everyone else to, too.

I don't know anything about James Brown. I don't know anything about the world of fashion, except that it has about as much appeal to me as the world of Apache helicopters. I don't know if it's common for people in the world of fashion to start talking about their cousins' sex lives, or their brothers, or somebody else's brothers, but I do know that they're not the only people in our society to be confused.

We're confused because we seem to think that to be black is to be "cool". We seem to think that being black has something to do with playing sport very well, or being very handsome, or very beautiful, or very sexy. We seem to think it has something do with multi-millionaire musicians who make music that uses words like "nigger", "bitch" and "whore". We seem to think, or some people seem to think, that knowing a black person, or having had sex with a black person, is something to boast about to another black person, or even to a white person. Something that will make us look "cool".

That, presumably, is why profiles of Samantha Cameron often refer to her friendship with a musician called Tricky. The friendship, like the dolphin tattooed on her ankle, is meant to be a sign of her street cred. Tricky himself doesn't even remember meeting her, though he has said that "the dates and times she mentioned match up". For Samantha Cameron, it was clearly quite a big deal to be playing pool with a black man. It was such a big deal for her husband to meet one that he mentioned it during one of his election broadcasts. You don't mention meeting someone black if you mix with black people all the time.

God only knows what Barack and Michelle Obama would have made of an ad that compared one of the most famous black women in the world with a bar of chocolate. God only knows what they'd have made of a racist rant at an awards ceremony where not a single person who heard it spoke out. But we don't need God to tell us that the world's most powerful man thinks young black men should "pull up their pants". We can probably guess that he's not too keen on people who use the word "nigger" or "whore".

It is, of course, fairly unlikely that Britain will get a black head of state. It's looking pretty unlikely that it will get a black prime minister, or even a black (not Asian) member of the Cabinet. But it would be nice if it had more black teachers, and doctors, and lawyers, and editors, and professors. (Out of 14,000 professors, only 50 are currently black.) Maybe then we won't get white people talking about black people as if they're another species. A species that Prince Philip might call "exotic".

"This," said Naomi Campbell's mother, "is the 21st century, not the 1950s. Shame," she said, "on Cadbury." Yes, shame on Cadbury. Shame on Brown. Shame on us.

c.patterson@independent.co.uk; twitter.com/queenchristina_