The news that a film is being planned about the late Isabella Blow, possibly with Sam Taylor-Wood directing, isn't a surprise. There was something inherently cinematic about Blow, who died in 2007 at the age of 48.

"Fashion's nutty aunt," as she was called by New York Magazine, Blow often dressed as if she was appearing in some high-society version of The Cabinet of Dr Caligari directed by Tim Burton.

The former fashion editor of Tatler was credited with kick-starting the careers of Alexander McQueen and Philip Treacy, among others. She led a colourful and sometimes troubled life in a very rarefied world, so there are all the ingredients in her life story for a strong movie biopic.

Despite this, cinema's previous engagements with the fashion industry, make it hard to avoid a feeling of trepidation. The box-office success of The Devil Wears Prada notwithstanding, movie ventures into the fashion world have often proved very shabby indeed.

Of course, fashion and films have gone hand in hand from the outset. Audrey Hepburn and Grace Kelly wore Givenchy. Adrian, MGM's top costume designer in the 1930s and early 1940s, who dressed stars from Greta Garbo to Jean Harlow and Joan Crawford, was largely responsible for the wildly exotic glamour found in even routine MGM films of the era.

There is a natural symbiosis between the two worlds, but when they invade one another's territories problems begin. To understand why fashion movies are generally so insipid, it is instructive to watch one of the better recent films set in this overheated world.

Matt Tyrnauer's documentary Valentino: The Last Emperor has one quality that most fashion movies lack, namely emotional depth. Tyrnauer realises that for outsiders, watching glamorous people in glamorous settings blowing air kisses at one another can be a deadening experience.

The behind-the-scenes bitching at catwalk shows or frantic efforts to finish new collections don't, of themselves, make for decent drama.

Tyrnauer's film has an elegiac quality. As the title attests, Valentino Garavani is the last of a breed. He and his business partner (and lover) Giancarlo Giammetti belong to an era of haute couture that is already passing. They stand for glamour and painstaking craftsmanship. Valentino's most famous clients included Jackie Kennedy, Marella Agnelli and Babe Paley. The high-society women that writer Truman Capote described as "swans" invariably dressed in Valentino.

But the core of the movie is the relationship between Valentino and Giammetti. Tyrnauer insists the fact they are in the fashion world isn't the key factor and that they'd make a fascinating couple even if they were farmers. He admits, though, that the opulence of the backdrops helps.

"It just so happens that we have million dollar sets for this movie which are their own environment. They're art directors of their own lives. There is a Pirandello thing going on here. They live art-directed lives but they self-direct."

The scenes in the documentary between the two principals are infinitely more lively, comic and dramatic than any of those sequences showing Valentino's celebrity friends.

One of the problems with fashion movies is that it is a world that defies satire. You can't easily make fun of a subculture in which outrageousness and self-parody are already inbuilt.

Jia Zhangke's 2007 documentary Useless took a Chinese perspective and asked why there were so few big-name Chinese fashion designers in spite of China being the largest exporter of clothes in the world. Zhangke's probing, thoughtful approach was the antithesis to that found in most fashion films - and perhaps explains why his film disappeared so quickly.

Zhangke's film was determinedly low-key. Menahem Golan's low-budget The Versace Murder (1998), a dramatisation of the events leading up to the killing of Gianni Versace in 1997, was sensationalist in the extreme, but it too struggled to find an audience.

When he directed Pret a Porter in 1995, Robert Altman was at the top of his game, having just made The Player (1992) and Short Cuts (1993). His intention was clear: to show up what he perceived to be the narcissism, media hype and shallowness of high fashion - a world in which, he clearly felt, sex, power and money matter more than any of the clothes.

One of the main characters is a gushing American fashion journalist played by Kim Basinger. She is utterly sycophantic toward the top magazine editors from Elle, Harper's Bazaar and British Vogue, who spend much of the film bitching about one another. To reinforce his point about so much in the fashion world being a case of the emperor's new clothes, Altman ends the film with a show in which the models pose naked.

Altman had a heavyweight ensemble cast that included Lauren Bacall, Sophia Loren, Marcello Mastroianni and Julia Roberts, all gadding round Paris during fashion week. The costumes were provided by, among others, Vivienne Westwood, Jean Paul Gaultier, Issey Miyake, Sonia Rykiel and Christian Lacroix. Even so, the film is neither engaging nor especially enlightening about the industry.

The challenge for film-makers is to make audiences care about a world that is predicated on its exclusivity and mystery, and whose rituals are often baffling. R.J. Cutler's feature-documentary The September Issue, ostensibly about the preparation of US Vogue's September 2007 issue, also offered a profile of sorts of the magazine's enigmatic editor-in-chief (and powerbroker extraordinaire) Anna Wintour.

Despite Cutler's best efforts, Wintour remained a stubbornly elusive subject. She is a sphinx-like presence who betrays little sense of what is driving her. Her impassivity is in stark contrast to the liveliness of her colleague Grace Coddington, who is as effusive, articulate and funny as Wintour is guarded.

Although it received respectful reviews, The September Issue was frustrating to watch. In spite of Cutler's privileged access, he wasn't really able to demystify the world he depicted.

Last year, we had Audrey Tautou in the picturesque but torpid costume pic Coco Avant Chanel. In 2007, there was Lagerfeld Confidential, a largely uncritical profile of the fashion designer Karl Lagerfeld. Currently in New Zealand cinemas we can see Jan Kounen's Coco Chanel & Igor Stravinsky, about the love affair between the two.

Chanel briefly went to Hollywood in the early 1930s to work for Samuel Goldwyn.

"Chanel has picked herself the hardest job she has ever tackled," the American magazine Collier's wrote.

"The world-famous fashion dictator who tells the duchesses and countesses and queens of Europe what to wear is now going to try and tell the duchesses and queens of the talkies. And it is just possible that the talkies may talk back."

Chanel didn't stay long. As Collier's had correctly anticipated, she didn't like being bossed around by actresses and her style wasn't considered extravagant enough. It must have irked her that designers were hired hands whose creative vision had little bearing on how films eventually turned out.

Tom Ford's success last year with A Single Man showed what fashion designers can achieve when they reinvent themselves as film-makers. Artists like Julian Schnabel and Steve McQueen have successfully directed movies.

There is a wealth of talent in the fashion world which could make the same transition, but they'll need to look beyond fashion for inspiration. Previous experience suggests that films about fashion aren't a very smart idea at all.