No matter what you do, don't rule out Burzum if you're going to write about black metal. I remember I was horrified when I saw Sam Dunn had put down Cradle of Filth as Norwegian Black Metal, and not Burzum 
Yes, I saw that too, they are not even Norwegian!... All joking aside, to me, Burzum is the most authentic, influencial and important black metal band out there.
Although I don't like black metal (aside from Filosofem and a few black metal-minded ambient artistst), I read about him, on his website notably, and the only reason why Sam Dunn did not mentioned Burzum was because of controversy. He sure knew the band.
The rest of his documentary was sure interesting but when he got into more complex stuff, death and black metal, he fucked up. Dickinson, Dio and Lemmy were great interviews but Lamb Of God and Slipknot... That was sad (remember when Slipknot said "You cannot think of an inverted cross without thinking of Black Sabbath" - they are Christian you dumbasses!). They have not credits for anything in metal whatsoever, but you guys know that...
Poor Sam Dunn who I though would make a good documentary when he started naming some of his favorite stuff (Beneath The Remains, Autopsy, etc.). Perhaps somebody hold him back on interviewing Varg Vikernes...