There's more to film noir than venetian blinds, sexy dames, and hard-boiled detectives in fedoras.
Heavily influenced by German Expressionism, the genre's visual style is probably its most recognizable characteristic, namely its use of chiaroscuro lighting. However, that's not the only part of the "black film" that requires our attention. Check out the video essay by Tom Elrod, publisher of The Critical Press, below to learn more about film noir's narrative traits, as well as how it relates to other Hollywood film genres.
In the end, it's not just the look that makes a movie a film noir, it's the story, too -- it's the narrative structure, the themes. Film Noir examines life on the wrong side of the tracks -- the crime, the filth, the women who land more on the side of the Whore than the Mother in the classic dichotomy. Most importantly, it examines what life would be like on the other side of the "everything's gonna be allllllright" coin. For example, if A Christmas Carol were a film noir, Tiny Tim wouldn't end the thing with "Merry Christmas, everyone!" Tiny Tim would be dead -- most likely. (Not sure why a Dickens reference came to mind, but there you go.)
If you want to take a look at some real film noirs -- not just the ones I come up with in my head in the middle of the night, there are a bunch of classics streaming on Netflix right now, including Double Indemnity, Raw Deal, and Scarlet Street. However, you should certainly check out The Maltese Falcon, The Big Sleep, or Mildred Pierce (which is a little bit of a departure from a classic film noir, but that might actually help to pinpoint its characteristics).
Which films/books/resources would you recommend to someone looking to learn more about Film Noir? Let us know down in the comments!