To those who picked Master's Hammer - Ritual, it was just re-released through NWN! as a 4x Vinyl. I've actually not heard them myself strangely enough, and was planning on doing so with this re-release.
Ok, my list (very difficult to make a list of 10):
Bathory - Blood Fire Death
Bathory - Under the Sign of the Black Mark
Burzum - Det Som Engang Var
Burzum - Hvis Lyset Tar Oss
Branikald - Av Vinterkald (Or Forest s/t, As a Song in the Harvest of Grief)
Dissection - The Somberlain
Enslaved - Frost
Inquisition - Invoking the Majestic Throne of Satan
Mayhem - De Mysteriis Dom Sathanas
Peste Noire - La Sanie des siecles - Panegyrique de la degenerescence
Notes: I'm still not entirely happy with Attila's vocals on DMDS (sorry), but i've come to grips with the fact that Dead was not on this...and lets face it, the music itself is killer, so it's definitely one of the 10. If this was a thread about 10 essential BM albums though, I'd be Live in Leipzig. Studio album? No. Most essential though? Yep.
Blood Fire Death - call it thrash, call it black, call it black/thrash, call it proto black, I don't care, it's definitely on this list.