Yah, "hard-boiled noir = mystery" is about as fair as "cyberpunk = speculative fiction." The Hobbit is not cyberpunk, and Dorothy Sayers didn't write noir.

The thing that's hard-boiled is not the noir but the detective. If you label a detective/mystery novel hard-boiled, you've got a detective who is likely to declare that he doesn't take nothin' from nobody, see? and no broad is going to trick him again. He'll come out shooting, dollface, because it's a cold, cruel world out there, and you gotta look out for number one. (Then he will go home and drink a lot and hate himself for it. The male pronoun is deliberately chosen here: there are now female hard-boiled detectives, but it started out very much male and sexist.)