Noir is about the atmosphere of the film. The token to me is that it is generally referring to a sort of American film, in contrast to the typical American film which is full of light and laughs and the heroes winning and getting the girls and, in contrast to other cultures, finishing the film alive.
The atmosphere in which the hero may not reach the end (DOA) or may be star crossed by his own actions (Detour) is rather different from a standard Western, Gangster, Fighter, War flick. Using a French term to apply to an American flick is an indicator.
I remember hearing that between the Wars, when Hollywood exported a film to Russia, they used a different final real, in which they killed the hero, this was called 'putting a Russian ending' on the film.
Noir was doing the same thing domestically, putting the whole thing into an atmosphere of dystopia. there are other signifiers, such as the role of women, not necessarily heroines, and cops.

I would say that White Heat, a terrific film, does not qualify as noir. High Sierra also a great film, does not qualify.

I think it can be argued that something like "X-Files" qualifies or at least owes a lot to noir in so far as theme, casting, and music are concerned.

Noir can have humor, but it always has doom.