Any colour, so long as it's black

Film noir is a 50-year-old idea. Yet two of this year's best movies belonged to that genre. And there are more to come. Why? By Ryan Gilbey

Despite the fact that its validity as a genre continues to be challenged in some quarters after nearly 50 years, film noir has proved to be a more enduring and pertinent phenomenon than Nino Frank, the critic who conceived of it, could ever have imagined. And it's growing. In the past few years, noir has permeated the darkest corner of every neighbouring genre, from science-fiction (Blade Runner) to horror (Angel Heart) and latter-day screwball (Something Wild). There have been imitations both flat (Romeo Is Bleeding) and lurid (Basic Instinct). And at least one director has built a career out of subverting its conventions - John Dahl, whose films (including Red Rock West and The Last Seduction) represent one Double Indemnity fan's determination to pay homage for as long as it takes to match the perfection of the original.

Two of this year's most acclaimed films, The Usual Suspects and Kiss of Death, also utilise prominent noir elements (the latter is a remake of a 1947 film noir). And there's more to come. The next six months will bring at least one bona fide film noir - Carl Franklin's film of Walter Mosley's novel Devil in a Blue Dress - as well as countless others underpinned by mainstays of the genre, including the Brad Pitt thriller Seven. So how has noir continued to remain thriving and pertinent? And has the genre splintered into too many fragments to render it as anything but dissolute?

When he introduced the phrase "film noir" into the vocabulary, Nino Frank was searching for a way to define a new strain of Hollywood cinema which emerged in Paris in the months immediately after the Second World War. These films - including The Maltese Falcon, Double Indemnity and Murder, My Sweet - were characterised by extreme visual and psychological departures from classical Hollywood cinema.

Visually, they displayed similarities to German Expressionism. The camerawork incorporated extreme high or low angles. Close-ups were tight to denote claustrophobia. Chiaroscuro lighting added to the effect, often affecting prisons of light and shadow to mirror the moral cages that film noir characters strayed into. The action unfolded in narrow corridors and cramped offices. When a film did venture outside, the exteriors invariably proved more constricting than the interiors.

The characters adhered to a similar blueprint. As the critic James Damico noted, the typical protagonist was "a man whose experience of life has left him sanguine and often bitter", who goes on to meet a "woman of similar outlook to whom he is sexually and fatally attracted". In some cases he "comes to cheat, attempt to murder or actually murder a second man..."

This plot convention, displayed in such films as The Postman Always Rings Twice (1946), was crucial in establishing a new kind of moral order which set film noir apart from the moralistic Hollywood cinema that had preceded it. The genre blurred the distinctions between good and evil until, by the time of the noir-influenced Dirty Harry (1971) (and, more explicitly, the 1984 thriller Tightrope), you needed a badge to distinguish cop from criminal. "My husband believes you to be an innocent man," Faye Dunaway assures Jack Nicholson in Chinatown (1974). "I've been accused of being a lot of things," he replies, "but never that."

The new heroes, it transpired, could appear shady, even villainous, from a particular angle; they were denied the linear routes of Holmes or Poirot, and were left instead to grope through the fog, their investigations lurching in fits and starts toward the truth, which would fit together in the final reel, if at all. If audiences were bewildered by The Big Sleep (1946), Chinatown and Miller's Crossing (1990), it didn't matter, because so were the heroes.

Terrence Rafferty of the New Yorker has pointed out that films noirs are less important for their plots than for the kind of classlessness that they expose, with the detective employed "as an instrument to reveal the elusive links between the highest and lowest levels of society... he often discovers that the guns of the underworld have been doing the dirty work of the wealthy and powerful". This coincides neatly with our appetite for scandal: the proof that, rich or poor, we are all bound by the same impulses and weaknesses. In Blade Runner (1982) it even extends to the creator/ creation relationship, in which the latter kills the former in an evocation of the Oedipal impulses which litter sexual relationships in film noir.

If this smacks of Freudian theory, then that is no coincidence. The emergence of film noir coincided with, and reflected, many significant events in American society - it responded to a rocketing crime-rate with a ground- breakingly subjective view of the criminal mind; and the strict Hays Code forced film to bury or disguise its sexual content. But the biggest influence on the thematic concerns of the burgeoning genre arose from the popularisation of psychoanalysis during the same period.

Frank Krutnik, author of the noir study In a Lonely Street, believes that Hollywood's preoccupation with Freud tallied perfectly with its required adherence to the Hays Code. Freud wrote of submerged symbols charged with meaning; the Hays Code unequivocally demanded that sex be hidden; these two factors enabled "a more elliptical and displaced mode of representation which could be `decoded' by audiences familiar with popularised psychoanalysis". The critics Raymonde Borde and Etienne Chaumeton described film noir as possessing an atmosphere of "latent, vague and polymorphous sexuality which everyone could project their desires into... like a Rorschach ink- blot".

It's hard to appreciate the level of subversion on which these clues operated now that Freudian symbolism has become such a commonplace language in modern cinema. Which may be why The Last Seduction (1993) is the best modern noir - it channels its heat and passion back into dialogue and casual physicality, just as Gilda and Double Indemnity did. It obeys the rules of film noir, which dictate that women are bad news for men. Yet it subverts those rules completely by making a woman the focus, and celebrating those very characteristics - greed, promiscuity, amorality - that film noir traditionally punishes in its females. (Dahl even allows the femme fatale to escape without paying for her crimes.)

The movie proves that film noir can continue to survive by re-interpreting the basic conventions to make them relevant to modern society. Placing a tough, heroic woman at the centre of Blue Steel (1989), for instance, sheds a different light on notions of masculinity which slumber at the heart of film noir. And the self-questioning of Blade Runner and Angel Heart (1987) indicates a further smudging of the boundaries between good and evil. Mickey Rourke spends the latter searching for a missing soul who, it transpires, was him all along; Harrison Ford's quest in the former is to eliminate Replicants, but he is unable to eliminate doubts about his own authenticity. These are classic noir themes - self-doubt, fallibility, the merging of hunter and prey - re-worked to make explicit enduring theological concerns that have always haunted the genre. That these themes have reached a point of implosion with The Usual Suspects, in which identity itself is exposed as fluid and fabricated, means that the next noir picture will need some very tricky cards up its sleeve.

But the permutations seem infinite. In its dialogues on sexuality and endlessly shifting gender roles, film noir may be more topical now than it has ever been. And if the essence of it truly has dissipated, leaving "pure" works like The Last Seduction thin on the ground, then we should be comforted by the fact that, in fusing with other genres, film noir has actually come full circle to its origins. According to the writer Paul Kerr, the initial noir B-movies were first conceived as unusual hybrids which could support an A-feature without being merely a dim shadow of it. They were "Poverty Row hybrids, mixtures of melodrama and mystery, gangster and private-eye, screwball comedy and thriller". Borde and Chaumeton argue that noir began as a marriage of "the horror film, the gangster film... and the social-problem film genres of the 1930s". More than 50 years have brought us no closer to a conclusive definition. That debate continues is in itself testimony to noir's intangible properties - and continuing rude health.

n `Devil in a Blue Dress' shows at the London Film Festival on 8 November. A Channel 4 `Noir' series begins on Sunday with the documentary `Cinefile: Dark and Deadly'