As I said before, it makes some sense to call those bands black metal, just as it can make sense to call Aerosmith and Led Zeppelin heavy metal, or Slayer and Exodus death metal, especially since bands like Destruction were at one point the closest thing to black metal. That's a matter of influence and ancestry, but it's also relative. In the 1910s, American progressives supported radical proposals like a 3% income tax, which would put them far to the right of any currently-serving Republican senator or congressman today. Those people don't get to use the term anymore. Metal genres become established when they stop evolving and settle on a particular set of riffs.
Sarcofago pushed the edge of what it meant to be a black metal, but it's not like they walked down to the polls and registered themselves as members of the black/thrash party. They upped the blasting, upped the shrieking, upped the tremolo-picking, etc, it was a gradual evolutionary process until black metal reached its quintessence. It was those things that earn them credit as a black metal pioneer, not the fact that they happened to still use some standard thrash riffs too, just as it's not the Diamond Head and Angel Witch riffs in Metallica's discography that established them as the seminal thrash band.
The exact day/year/release doesn't matter. I can accept Hellhammer/Celtic Frost being black metal proper, sure (I don't consider them to be a thrash metal band at all). I think there's a clear musical line that began to be drawn by the time of post-Deathcrush Mayhem, however.