You can enjoy a free version of Bill Zebub’s black metal documentary on Tubi https://tubitv.com/movies/714342/black-metal-and-other-dark-music

It is still long (at over 2.5 hours) but shorter than the 7-hour version on Bluray (BLACK METAL: THE ULTIMATE DOCUMENTARY). Perhaps you can consider this as a try-before-you-buy, and when you DO purchase the Blu(e)ray, then you will get many more hours of new content.