First of all, welcome to Katatonia forum.
You know, it is something special being a fan of Katatonia, and concerning the 'term' for describing the music they play...
Well, it's...Katatonia! You just can't call it anything else. I mean, it would be unfair just to call it 'metal', or 'dark metal' or 'heavy alterock' etc.
In Viva Emptiness (and Katatonia albums in general), you can hear so many influences: Slowdive, Pink Floyd, Jeff Buckley, the Cure, Nick Drake, Radiohead, the great feeling of 70s, the Great Doom of the 90s(PL, Anathema, MDB), Portishead, great guitar sound in general...
the same is with bands such as Opeth, Tool, Anathema, the Gathering.
How WOULD you call them? Exactly like they are - Opeth, Tool, Anathema, the Gathering.
Hope I didn't disappoint you, but that's the way it is(or at least the way I see/hear things).
cheerz