So you've already ordered "In the Nightside Eclipse." That's a good start.
I'll include heathen/NSBM stuff here, because it's easier. Same musical roots as BM anyway. Get any/all of the below:
Mayhem "De Mysteriis Dom Sathanas"
Ignore those who whine about the vocals. The music on this is influential like no other BM album. It has amazing atmosphere and riffage, no filler tracks. One of the greatest BM albums ever... but yes, I would have liked to hear it with Dead on vocals. Any MAYHEM released after this can be safely skipped.
Burzum "Hvis Lyset Tar Oss", "Filosofem"
I don't even know how to describe these... "Hvis Lyset Tar Oss" holds the most amazing piece of music I have ever heard... You'll either hate BURZUM (like Dreamlord) or you'll be absolutely entranced by its obscure beauty.
Enslaved "Vikingligr Veldi"
Their best in my opinion, epic stuff with immense atmosphere... Yes, they call themselves viking metal but it came from the same scene as Mayhem, Immortal, Darkthrone and all the other Norwegian bands and does sound like it.
Helheim "Jormundgand"
Read my review of it on RC, that about sums it up.
Bathory "Blood Fire Death", "Under the Sign of the Black Mark"
Early BM, but more intricate and progressive than you might believe. UtSotBM was certainly very progressive in its day, with all the synthesizers and stuff. Not only fascinating history lessons but downright amazing albums in their own right.
Darkthrone "Transilvanian Hunger"
I think my review of this on RC also pretty much covers my feelings about it. A must-have, to say the least.
Satyricon "Dark Medieval Times"
I never was the greatest Satyricon fan, but this is one of those great legendary Norwegian albums that must be owned, complete with immense atmosphere that many have tried and failed to copy...
Immortal "Diabolical Fullmoon Mysticism", "Pure Holocaust"
Hm. Great stuff. Immortal at their best. The former is actually kind of mid-paced for the most part. Wonderfully obscure and atmospheric in production and has a twisted catchiness to the songwriting. The second is probably the best of their hyperspeed albums, and universally hailed as a genre classic.
Somewhat less legendary/essential but still excellent stuff:
Nokturnal Mortum "Goat Horns"
Ukrainian folk influences, melodic, cold... Excellent. Actually, everything this band has released is worth getting.
Dissection "Storm of the Light's Bane"
Not purely black metal perhaps... You probably know Dissection already?
Kvist "Før Kunsten Maa Vi Evig Vike"
Criminally underrated gem of keyboard BM. Great, great album, but there is no information anywhere on the band -- not even member names -- and this is all they ever released. Mysterious.
Taake "Nattestid Ser Porten Vid"
One of the best Norwegian bands still around. The follow-up album is also great. Should appeal to you as an Ulver fan alright.
Carpathian Forest "Through Chasms, Caves and Titan Woods"
EP, short but great in the style of later Darkthrone (before Darkthrone sounded like that) with some more "atmospheric" bits thrown in. Actually the whole thing has great, twisted atmosphere.
Manes "Under Ein Blodraud Maane"
Unique... A lot of synths but very epic and desolate-sounding. Has recently been re-released.
Graveland
I'm no Graveland expert so I can't tell you which album to get. You should definitely try them though.
Also, any Negurã Bunget. You should've gotten the hint on Negurã by now...
Uh... That should get you started
