I think that Noir is much more about the style and "feel" of a movie, whereas genre movies fit quite nicely into what the setting is, e.g. sci-fi, horror, etc.
But you can get sci-fi noir (Bladerunner) & Horror noir (Angel Heart), etc.
The archetypal film noir (if there is such a thing) seems to be centred around the 1940's, whether made during that period (Double Indemnity) or set during that period (The Man Who Wasn't There). Interestingly, the concept seems to be a bit more hit and miss when given a contemporary setting, e.g. The Last Seduction (hit), After Dark, My Sweet (miss).
Perhaps rather than being considered as a genre, noir should be either considered a style or a period.
I think noir is by far the widest ranging cinematic collectives which gives it a fantastic depth, but also makes it incredibly hard to define. Which I guess is why I like it so much.