Black metal wasn't strictly satanic even at the early stages. It's well known, that Burzum had nothing to do with Satanism and even with Paganism on the first 4 albums. It had a pretty abstract concept of darkness. Black metal was created not only as anti-christian movement, it had anti-human concept and some other "anties" from the beginning. The religious context had developed to a major one, but still the ideology had never been that narrow.
Very early bands like Venom, Hellhammer, Samael and Bathory were Satanic. Of course Bathory evolved into Pagan/Viking. Note that bands like Mercyful Fate/King Diamond, Destruction, Death SS, Slayer, Morbid Angel, Possessed etc. inspired BM as well and they were Satanic. Actually the debut of Burzum and ''Aske'' are kinda Satanic but Burzum was more about darkness and Paganism, while Mayhem was more into theistic Satanism.