well Opeth are generally called prog metal, but you're on the right track - bands like Burzum, Emperor, Darkthrone etc are in that sort of category.
Theres a fair bit of debate about what is 'tr00' BM, generally there is the scandinavian type stuff, then NSBM, which has a more overtly fascist sort of bent, then theres 'viking' metal to do with norse legends and all that sort of stuff, black/thrash along the lines of Destroyer 666, and it gets even more complex when you start talking about genre's such as melodic black and so on.