In order to understand better what is going on here we need to know/remind the basics. Yeah, its about Heavy Metal here: long hairs, chains and leather, loud guitars, offensive album covers and usage of swear words. This is how some heavy metal subgenres were autopsied in the 80s (taken from Muusik #3 1990):
THRASH METAL - extreme fast, extreme heavy and extreme blurry heavy. Arose from the mixture of heavy and punk.
SPEED METAL - somewhat close to thrash but more with rafined sound and structure. More melodic.
DEATH METAL - close relative to black metal and thrash metal. Extended usage of death-related in the lyrics.
BLACK METAL - Venom started it. In most cases the music goes in the thrash vein, lyrics are full of satanism and black magic.
DOOM METAL - slow, doomy and dark.
I like those descriptions. Death and Black Metal were not standardized back then yet (now Death Metal is connected almost always with Cannibal Corpse type of brutal thing (nothing against CC, just an example), and also Black Metal is connected with high pitched shrieks and muddy shitty sound. And Venom and Mercyful Fate are not considered Black Metal anymore by most new people. But who cares anyway about such labelings...