Noir fiction is the name sometimes given to a mode of crime fiction regarded as a subset of the hardboiled style. According to noir aficionado George Tuttle,

In this sub-genre, the protagonist is usually not a detective, but instead either a victim, a suspect, or a perpetrator. He is someone tied directly to the crime, not an outsider called to solve or fix the situation. Other common characteristics...are the emphasis on sexual relationships and the use of sex to advance the plot and the self-destructive qualities of the lead characters. This type of fiction also has the lean, direct writing style and the gritty realism commonly associated with hardboiled fiction.[1]

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The popular use of "noir" in the term "noir fiction" derives immediately from "film noir" as it has been used to characterize certain putatively "dark" Hollywood crime dramas and melodramas, many early examples of which were based on works by the original hardboiled writers. In turn, "noir" (French for "black"), first applied to American films in the mid-1940s by observers in France, was used there in similar senses. Most relevantly, the term roman noir ("black novel") was employed to describe a range of books, some that an English speaker might think of as mysteries, others as gothic melodramas. Note that while the meanings of "noir fiction" and roman noir are closely related, the derivation is not direct. Making the connection even tighter, in 1945 the French publisher Gallimard brought out a new series of paperback thrillers, many of them translations of hardboiled American fiction. The line was called Serie noire.

It's too easy to get bogged down in the requirements and boundaries of genre fiction (but I'm doing it as I write)

Clearly, hardboiled and noir crime writers have something in common with the existentialists, and it's probably not an accident that both movements flourished in the aftermath of World War II. Both have a grim view of life as painful and not subject to extrinsic guarantees of justice or fairness. There is no god in an existentialist universe and the only justice one finds is the justice one makes for oneself (if it's even possible to do so, which it isn't always).

But is there more to it than that? What is the connection between existentialism and crime fiction, and how do the two inspire each other? And why is it that fifty years later crime fiction is still going strong while you'd be hard pressed to name a writer who calls himself an existentialst?