Traditionally, there's an amount of melodrama (clear good/bad), a femme fatale (a woman who brings about the doom of the man), a distinct visual style, etc. Or so I was told in a couple of film classes.
I would add a general attitude of cynicalism, certainly on the part of the lead character.
I don't know if strictly speaking the film would have to be in black and white, but visually it should be "dark" and muted.
Raining the whole time (Blade Runner) helps.