Burzum is not really "classic" black metal (in fact all three of those bands are to some extent from the 90s wave of black metal) - and at times not really much like black metal either. Large parts of Hvis lyset tar oss and Filosofem are ambient, and many of the songs which aren't remind me more of My Bloody Valentine than Bathory.
But I do agree that the vast majority of black metal was shit, even more so in the first wave than in the second. So's the vast majority of everything. Even Burzum is about two good albums, two albums which are about halfway good, and three or four which are crap. That's to be expected in a genre which revolves around utter madness, nihilism, brutality, and other such cheerful things. The quality of the music corresponds inversely to the security of Varg's sanity, which may be which Burzum has mostly musically irrelevant and not particularly good since 1996.
Darkthrone aren't remotely black metal anymore (and Fenriz was always far more interesting for his thoughts on the black metal scene than for his contributions to it) and Mayhem were shit for literally a whole decade before Ordo ad Chao...these days traditional black metal is kind of dead, it's all in the Viking stuff these days. (Not to malign Viking metal and the like, because Ensiferum, Turisas, Tyr, Moonsorrow and probably a lot of other groups I'm forgetting are enjoyable as hell - but black it isn't).