Well, as far as themes go (not just visual concepts), the definitive noir film must address the bleaker aspect of human nature, with a common mood of cynicism, distrust, paranoia and guilt... There doesn't have to be a femme fatale, though one would be welcome. "A divorced cop, with a murky past, hires a contract killer to exterminate his ex-wife but gets more than he bargained for when he falls for the gun-for-hire's seductive wife" would be a definitive noir plotline. Guilt, hopelessness, lust and murder all come into play.
Remember film-noirs continue to be produced today. They're called neo-noirs. Body Heat, Memento, Drive, Taxi Driver, Brick, American Psycho, the list goes on, etc, etc.