I've watched a lot of 1940s/early '50s Westerns recently. They all are, in essence, films noir with a historical setting. And there are a lot of them.
Budd Boetticher's Westerns all have strong noir elements. Horizons West (1952), The Cimarron Kid ('51) and The Man From The Alamo ('53) all grapple with the theme of a man assumed to be a criminal, and/or is forced into a criminal life. Society paints this character into an inexorable corner. The only way out is to commit crimes, and to lead a conflicted double-life while doing so.
Or course, The Boetticher "Ranown" Westerns with Randolph Scott also have strong noir tendencies. So do the '50s Westerns of Anthony Mann and late '40s efforts like Silver Lode, Ramrod, Pursued, Yellow Sky, Blood on the Moon and Station West.
This isn't exactly a new school of thought. Silver and Ward's noir encyclopedia acknowledges the noir/western fusion in one of its appendix sections, and notes several of the movies I mention above. I think there are enough of these western noirs, and enough of them are of high quality, that this branch office of the genre deserves more recognition and consideration.
As well, for those who have "seen it all" in classic noir, that these post-war Westerns offer an intriguing, rewarding filmic vein. Some of the same major noir cinematographers (John Alton, Burnett Guffey) worked on these, and, as Westerns were more likely to be color films, they show how these visual artists adapted noir techniques to a very different set of rules.
I'd like to hear what other Back Alley denizens have to say about this topic...