It's sometimes noted, quite correctly, that the artists who made what we now think of as the classic films noirs were entirely unfamiliar with the term, and indeed had no conception that they were working in a distinct tradition.  They thought of the movies they were making as crime thrillers.

This is occasionally cited in support of the idea that the term film noir is a category created by cinéastes after the fact, and therefore inauthentic, misleading.  It certainly was created (or at least popularized) by cinéastes after the fact, but that doesn't mean it's inauthentic or misleading.  Such a view fails to take into account how genres and traditions arise, which is a complex process — a combination of historical and cultural trends, influence and imitation among artists, and simple commercial calculation.  All these factors can combine to create distinct new forms, and in the case of film noir I think they did.

Two early films, which I would not call films noirs, nevertheless set the tone for the new form — The Maltese Falcon and Double Indemnity.  The Maltese Falcon was a fairly standard work of hardboiled detective fiction but it had a twist.  In hardboiled detective fiction, the world might be a dark and messy place, but the detective had a code of honor which made a kind of grim moral sense amidst the darkness and the mess.

Sam Spade in The Maltese Falcon had such a code and he stuck by it — but Huston allowed him more than a trace of doubt as to whether the code had any ultimate meaning, any ultimate value.  This was something new in the crime thriller, in hardboiled detective fiction — this hint of existential uncertainty.

In Double Indemnity, essentially a domestic murder melodrama, Billy Wilder offered a portrait of middle-class American life that was unremitting in its bleakness, its moral vacuousness.  I'm not sure that Wilder had any particular message to convey by this — he just sensed that in the midst of the global horror of WWII audiences were looking for sterner stuff in their melodramas, a darker vision of ordinary life that would accord with the experience of civilization as a whole gone suddenly mad.

They didn't think, “We're going to create a new kind of existentially challenging crime thriller.”  They just inflected the crime thriller with a new mood.  Audiences responded, and formulas began to solidify.  Film artists imitated each other, got turned on by each other's work.  Elements that worked in one film got incorporated into other films, given new twists.  It was a combination of playing it safe commercially but also pushing things as far as they could go within familiar territory — testing how much darkness the public really wanted.

It turned out to be quite a lot — so much so that that during the Fifties filmmakers began to realize that the darker themes could be incorporated into other genres besides the crime thriller, as they were, for example, in the domestic melodramas of Sirk, in the Westerns of Ford and Mann.  When that happened, the classic film noir more or less played itself out.  Its usefulness as a cultural escape valve had ended.  Any kind of film in the Sixties could deal with existential angst, with moral bewilderment, with political or social criticism, in more direct terms.  America had internalized the darkness of the film noir — the resulting culture wars were just a matter of time.

Film noir had a beginning in the global dislocations and moral derangement of WWII, and an end in the open social and political critiques of the Sixties.  There had never been anything quite like film noir before WWII, and there has never been anything quite like it since the Sixties.  It was, and remains, a distinct tradition.