Film Noir - A style not a genre
Film noir is a set of achetypes that a film utilizes for developement. A genre of film is a class of ideas on which the film is based(what it's about). Genre = Westerns, Romance, Documentary. Style = film noir, slapstick, Claymation.
When describing movies that are shot in black and white, lots of people say that they are film noir, in a sense yes they are, they are usually dark because of poor lighting. The things that must show up in a movie to make it a film noir are: a doomed love, a portrayal of the cruelty of human nature, pointed dialogue, heros that are driven by past mistakes, "dames", and anxiety/pessimism/fatalism.
And the easiest way of differentiating these two is that when describing a film, film noir can be used as an adjective and a genre cannot. Behold, a film noir love story, right!, a romantic film noir story, Wrong!