1. Venom came out of the NWOBHM scene. Historical fact. They were actually a transitional band that helped to refine the genre of thrash and heavily influenced death and black metal both. If one labels Venom black metal, then, out of fairness, they must also label Mercyful Fate (another primary influence on the genre) black metal.
2. DMDS is the only Mayhem album in my opinion that is a classic, the rest of their discography ranges from mediocre to just plain terrible. And I prefer Emperor, Burzum, Immortal, and Darkthrone to Mayhem. Also, and I know I've said this before: ATTWAD > ITNE. To confuse matters further, I might venture to assert that WOTT is the best of all Emperor recordings (though I don't always feel that way).
3. As for Deep Purple, Led Zeppelin, and Uriah Heep, keep things in context. In the early days of the genre, of course they were metal. There was no other genre befitting of those bands back in the 1970's, even if they could be categorized as nothing more than rock these days. Remember, Slayer was labeled death metal back in the 1980's.