Listen to A Blaze in the Northern Sky and Panzerfaust. Those albums are more than just influence. They use direct aesthetics that Celtic Frost use.
First off, aesthetics, not actual musical structure. If I record a pop-punk album in my basement with ridiculous reverb and shitty mics, is it now black metal?
Secondly, are you aware that Darkthrone's first album is death metal? They're hardly the best example of pure black metal.
Thirdly, if Emperor had ripped off Metallica riffs, would that have made Master of Puppets black metal? No, it would have meant that Emperor were incorporating thrash metal elements into black metal or at most were playing black thrash.
Let me put it like this: If A comes before B, then regardless of what B does or says, A is still whatever A originally was. This shit isn't retroactive. This probably isn't very helpful, and is actually confusing and potentially flawed. I'll stop.
Since the Quorthon/Venom thing was mentioned: I believe Quorthon simply because it's such a ridiculous story. Who would do that? "yeah, so I'll blatantly rip off their sound, write some songs with the same names, some shit like that..." I also find that in fact Bathory drew more on what Quorthon cited - Oi punk - and Venom on whatever they were drawing on and while Black Metal and Bathory sound pretty similar if you listen to the riffs they're in fact not actually that similar.
But to me it doesn't matter, because Bathory's best stuff was after Bathory.