As detailed in The Heavy Metal FAQ, heavy metal developed through parallels found between several musical traditions both inside and outside of the developing genre. Black Sabbath emerged from the intersection between heavy rock, nascent punk and progressive rock; black metal emerged from proto-underground metal and hardcore punk, taking the most intense aspects of each and fusing them together.
The Until the Light Takes Us crew filmed a lengthy interview with Fenriz in which he outlines the roots of black metal in several stages, from its rise in heavy metal with Black Sabbath, Motorhead and Mercyful Fate through the expansion out of speed metal (via hardcore and thrash) through Slayer and finally, the proliferation of bands inspired by NWOBHM band Venom such as Sodom, Hellhammer and Destruction from which much of the modern sound emerged, finalized (in his view) with Bathory Under the Sign of the Black Mark.
As black metal rotates around the same carcass that stopped artistically expanding 22 years ago, understanding its roots becomes even more important as it is necessary to drive away the ghouls, parasites and grave-robbers desperate for some of its legend through imitation and pandering to a new audience. The two videos add up to about fifty minutes of air time and worth watching for the black metal fan, historian or curious bystander.