I typed up a much more rambling version of this and then refreshed and saw this! Anyway, yes, this--I agree that it's (annoyingly) an umbrella category as well as a category of its own, which was probably inevitable because that's how bookstores and libraries tend to classify them.

It's funny that movies don't tend to use mystery as an umbrella term. There are also different standards for labeling a book as noir vs. calling a movie noir. Classification geekery is both ARGH and kind of fun.