Originally Posted by The Batlord

Trying to make it cut-and-dry is pointless. By the nineties, thrash, death metal, and black metal were distinct genres, but throughout the eighties the distinctions between them (not to mention grindcore and even hardcore) could be so vague that classifying them is only useful in hindsight. Sarcofago was somewhere inbetween all three, early Sepultura likewise. Even early Slayer could just as easily be called first-wave black metal. Other bands that are called thrash, such as Sodom and Destruction, have early albums that are considered black metal. Obituary has a heavy Celtic Frost/Hellhamer influence the same as the Norwegian black metal bands. Listen to early Death demos and you can see a straight line going from those same influences until they finally record their debut which is death metal that's still not removed from thrash.

When genres are developing they're just too linked to multiple other genres for classification to be black and white.