If I might take issue with that, The Black Dahlia is based on separate novel by James Ellroy about the real life murder of aspiring actress Elizabeth Short, the first of his LA quartet and filmed by Brian De Palma with, by all accounts, rather mixed results. LA Confidential is a the third novel in the quartet and is mostly fictional, based on the vicious rivalry between assorted criminal factions to manufacture and distribute a super pure form of heroin. All of the quartet are worth reading if you're looking for inspiration for an ultra-noir campaign although I think the law of diminishing returns had started by the time of the last novel White Jazz. Pretty much a rehash of what had gone before and the beginning of Ellroy's descent into self-parody.
Brown's Requiem based on his first novel, and starring Michael Rooker is a fairly decent attempt at a modern noir movie.
There's also Cast a Deadly Spell with Fred Ward as a Marlowe-esque private eye named HP Lovecraft (groan) going up against David Warner's mythos sorceror in an alternative history Los Angeles where magic is common place. It's a bit too knowing and in-jokey to be a real noir, and rather lacking in existential bleakness, but is enjoyable enough on its own terms. Not seen the sequel with Dennis Hopper replacing Ward. Might be all right.
Thought of another one, Night and the City which is notable for showing the streets of post war London as being every bit as mean as those of New York or LA. In a similar vein I'm also tempted to mention Hell is a City and Payroll but these are perhaps closer to straight crime stories than film noir. Certainly elements of noir in both though, Payroll in particular has a rather doom laden atmosphere about it.