Someone who listens to black metal music.
Cradle of Filth are Black Metallers
Anyone who is into black metal. Black metallers often are known to appreciate Black Metal, norway, philosophy, classical music and sometimes Satan (usually it's just the band members who do the whole Satan thing.)
Although Black Metallers are considered nasty people to be around, they aren't often around due to the lack of live performances from bands. Very few bands perform live and when they do I can admit the fans go crazy but look at the anarcho-punk scene or the american rap scene? The only difference is the fact Black Metallers keep it far more underground.
Everyday joe: I don't like the idea behind this "death metal" music of yours
Black Metaller: It's called black metal and how can it be harmful if you never even heard of it?