It's nearly August, the retail fashion industry is in an uneasy slump, and summer issues of women's magazines are gaunt for want of advertising. Yet in the past four weeks, the 'black issue' of Italian Vogue has caused such a phenomenal demand at news-stands in Britain and the United States that Condé Nast, the publisher, has rushed to reprint and distribute 40,000 more copies.

The explosive content of what, by any standards, is a small-circulation magazine with an average monthly sale of 109,000, is now being spoken about as a cultural watershed in fashion. With the next show season six weeks off, its influence might finally end the 'white-out' that has come to dominate catwalks and magazine pages.

On Friday, a saleswoman on the till at WH Smith in Hammersmith Mall, west London, was proudly gesturing to a Vogue Italia propped up at her cashdesk. 'We've managed to get 10 more,' she said, as a group of black and mixed-race schoolgirls broke ranks in the queue and doubled back to the shelves, hollering with delight. They have reason to celebrate, and to hope. One of the covers of Vogue Italia features Jourdan Dunn, the 18-year-old who was discovered by a Storm scout at Primark in that same mall. Perhaps those girls were her former classmates.

Conceived by editor-in-chief Franca Sozzani, and shot by Steven Meisel from a roster of 18 new, established and former stars, the July 'black issue' sold out in Britain on arrival. That renowned fashion photographer Steven Meisel, the recluse whose lens has made the career of many a model (including Brits Lily Cole and Karen Elson) should be focusing on non-white subjects might have been expected to cause some debate. A mild examination of conscience among the model scouts, agencies, casting directors and designers was privately anticipated, without much hope of anything changing - in the same way that the endless skinny-model debate has resulted in little or no change in the industry. But no one anticipated the global interest.

'It has been unprecedented, a sensation, although that wasn't the aim,' said Jonathan Newhouse, chairman of Condé Nast International, who masterminded the reprinting and rerouting of unsold magazines in Italy to America and Britain. In America, the issue is shrink-wrapped and stickered with the words 'First Reprint. The Most Wanted Issue Ever'.

That is no hype, according to the Kenya Hunt, the young black style editor of Metro International News in New York. 'I've been watching the news-stands since the beginning. There are lines of women when they hear of a new shipment. It's a wide cross-section of women, girls, people my parents' age who read Ebony,' she said. 'There have been email chains about it. The news-stand guys are hustling, locking it up in the back and charging $25, $28, when the real price is $16. Yesterday, I saw it on eBay for $50. There is a climate shift. This is the year of the presidential election. And this at a time when magazine sales are really hurting.'

British retail newsagents, who are often reluctant to allot shelf-space to any non-populist publication, have also been scrambling to satisfy waves of people wanting to get their hands on the £6.50 edition. Borders has had to move issues around the country after a first-day sell-out, saying that demand was 654 per cent up on the previous issue. WH Smith's women's magazine head buyer Louisa Stokes confirmed: 'I took so many queries from customers and from individual store managers - I've not seen anything like it. Italian Vogue is normally delivered to only 45 stores, but customers all over the country are asking for it.'

The first printing was so scarce in London that Edward Enninful, the black Notting Hill-born Vogue fashion editor, who worked with Meisel to style the Naomi Campbell cover (one of three versions of the edition) was forced to scour the country for an issue. 'I couldn't believe it. I ended up phoning friends in Ellesmere Port in Cheshire, who found one for me in a corner newsagent. I am so excited. I never thought I would be able to see something like this - my people, my race, wearing the collections, being gorgeous, chic, real women in that way. But the most important thing is: this proves we are bankable. We can sell.'

That, of course, is the point. Evidence of commerciality, especially in anxious times, is more likely to shift industry thinking than any amount of political correctness. For years, the excuse proffered by advertising agencies for not signing black models to lucrative contracts, and by magazine editors who failed to feature women of colour on their covers, was a supposedly factual, 'they don't sell'.

Certainly, part of the grand rebuttal has been organised through the internet, influencing an industry that is far too used to listening to its own circle of insider voices.

Sola Oyebade, chief executive of Mahogany Model Management, has been running a Facebook, text and email campaign in an attempt to make the issue the biggest-selling Vogue ever. 'We believe there's industry apartheid and this is something that the black community does feel very strongly about. I've had so many calls from people asking where they can get their hands on a copy. I've had shops like Harvey Nichols ring me up, telling me we're causing them a lot of problems with supply, so we've been liaising with them, too. Our fashion industry is institutionally racist. The explanation for why they don't use black models is always that we don't sell, but this shows that's not true. Black people are among the highest consumer spenders in the UK for material goods.'

Agencies are busy scrambling to catch up. London-based, Nigerian-born designer Duro Olowu has fought to cast black girls in his show for the past three seasons. 'I'd phone agencies, and there would be silence. Now, people will have a responsibility to make sure they have black models in their shows,' he said. 'Anybody who doesn't will look an idiot.'

However, the truth is that it has taken two central establishment leaders - Sozzani and Meisel - to make the subject even discussable in fashion circles.

Just a few months ago, anyone who voiced opinions like Olowu's would have been regarded as naïve, eccentric or speaking out of turn and in danger of ostracism. 'I wrote a piece about the absence of black models a year ago,' said Kenya Hunt. 'and no one wanted to talk about it.' Now, she says, everyone is rushing to quote it.

As Sarah Doukas of Storm, the agency that discovered Jourdan Dunn and manages Alek Wek, put it, the success of the 'black issue' 'has implications for all of us to now fully embrace the diversity within our industry, and to exploit our creative resources to celebrate our cultural and social differences.'

It is, after all, a long overdue wake-up call for an industry whose precarious future will rely on reaching global markets that do not resemble the freakish army of half-starved six-foot white girls who have come to represent the Western ideal.

As Barack Obama has it, it's time for a change. Though it may seem forced to link politics with fashion, history proves that the dominant aesthetic of any era can only reflect the mood of the times. And just as in politics, what just might revolutionise fashion now is that enough white people, too, are bored to death and impatient with the way things look out there.

Speaking out

Jourdan Dunn, the teenage supermodel who was first spotted in a Primark store in west London, was interviewed in The Observer Magazine in April after she had criticised London fashion week - and its catwalks - for being too white. She spoke about race and role models ...

· On being spotted: 'Everybody says I was spotted shopping in Primark. I wasn't shopping, I was with my friend. She wanted to go in, I wanted to go home - and we were just mucking about in the sunglasses section.'

· On her comments about London Fashion Week and race: 'It needs to be said because I think about these things and other girls do, too.'

· On the reaction to what she said: 'The way people said I was stupid made me feel horrible, saying fashion's a business so they need to use models who sell things.'

· On Naomi Campbell's plan to establish a modelling agency to promote different races: 'Naomi's idea is good. I'd do an agency for black girls and Asian and Spanish, because there aren't enough of them on the runway either.'

· On posing: 'I like having spikes coming out of my head or being in something I'd never wear.'

· On being away from home: 'I miss out on getting on my brothers' nerves, so when I get back I have to get on their nerves on purpose to catch up.'