Derek... Mercyful Fate, Bathory, and Venom are all considered important inspirations for the black metal genre, as far as imagery and lyrics and speed is concerned.
To put it in a different context, think of the Sex Pistols and their inspiration to the punk genre. No one's going to argue with the fact that they're an influential punk band, but in all reality, their songs sound a lot different than the punk bands that were to influence. For one, there was actually some somewhat "technical" (comparatively speaking) playing on Never Mind the Bollocks.. not to mention all the songs were so damn slow (again, in comparison to the bands who followed). However, they were influential in their lyrics, attitude, fashion, and so on.... even if most punk bands don't really sound like them.
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