Genre
Satire; Tragedy; Dark Comedy; Film Noir
This one runs the gamut, Shmoopers. Imagine a movie that manages to be about twelve things at once, and you've got Sunset Boulevard.
Captain Obvious says we should start with, well, the one that's obvious. So that brings us to…
Satire
Sunset Boulevard satirizes celebrity and Hollywood—that's pretty clear. Norma Desmond, while having some of the traits of real people, is a caricature. She's over-the-top, an outsized version of a still-proud has-been.
Meanwhile, Hollywood appears to be a place that doesn't really value good writing—just stuff that'll sell. Joe Gillis acts as Wilder and Brackett's cynical mouthpiece, poking fun at producers who would've rejected movies like Gone with the Wind (because "Who wants to see a Civil War picture?") and would've rewritten scripts originally set during the Dust Bowl so they took place on torpedo boats instead. The Hollywood of Sunset Boulevard is, in short, filled with hacks.
Both Tragedy…
But it's a tragedy too. While Norma is a satirical version of the ultimate arrogant former star, Joe Gillis is a pretty normal guy… and we get the sense that Norma was more down-to-earth as well, once upon a time. A sense of tragic doom hovers over the movie, as Joe gradually sinks to his inevitable demise and as Norma tilts toward madness until she's finally, truly insane.
… and Comedy
And, at the same time, the movie is a dark comedy (closely related to the satire genre). We see Norma's pet monkey's funeral, we see her weird Charlie Chaplin impression, and we see her guilt-trip Joe into sleeping with her by attempting suicide (which isn't really funny, but it's kind of too weird and macabre to be wholly serious). There's humor in the absurdity, don't you think?
A Turn Toward the Dark
Also, you could call it a film noir, since it deals with a murder; has that eerie, shadowy, black-and-white vibe; has a down-and-out narrator; and takes place in Hollywood's weird fringes. That fits the film noir bill for sure.