When the Nazis took power in Germany in the 1930s, many German film-makers fled the country, as did many of the cameramen who worked with them. They settled in Hollywood, where their experience with German expressionism influenced the work they did on American movies, especially adaptations of hardboiled detective and gangster stories. Even when they weren't directly involved in a particular movie, they still had an artistic influence on those who were.
The “dark shadows” style of film noir owes a lot to this influence, as well as to the fact that a lot of the B-grade noirs were shot without adequate lighting due to budget constraints. A combination of artsy German cinematic techniques on the high end with “making the best of a bad situation” on the low end resulted in the defining style of American crime movies. When French film critics were finally able to view American movies again after the war, the thing that struck them most forcibly was the presence of heavy shadows, sometimes rendering a scene almost completely black- thus, “film noir.” Combined with a French tradition of short crime fiction known as “serie noir” for its dark subject matter, and the concept of the American film noir was born.
You can actually see a clear distinction between the noirs of the 40s, when the term had not yet been invented and directors were not conscious of working within a specific style, and the noirs of the 50s when they were self-consciously “doing film noir.” The noir films of this later period are much more self-conscious about it, layering on the shadows for an exaggerated effect. “The Maltese Falcon” is widely considered the first of the classic noir movies, and “Touch of Evil” the last. If you watch them both back to back, the difference is striking.