I've heard a couple of bands use lower tunings in black metal focused material and it's an interesting sound I haven't seen talked about or explored hardly at all. Anyone know of any bands that use lower tunings (like below C) or play 7-string guitars?

The Ruins of Beverast comes to my mind immediately, but that is about it. Panzerfaust maybe, although I do not know the actual tuning they use. I do not remember to have ever seen a 7-String guitar when watching a black metal band live.

Batushka use 7 or 8 string guitars

OP should check out Demoncy's "Joined in darkness" then, one of the very best US-American black metal albums.

Barathrum, they use like three bass guitars an just one standard guitar so the riffs sound pretty low.
Necromantia is another band who only uses standard guitar for solos, but the rest of the music is just bass.

Not sure of the actual tuning, but Void Meditation Cult sounds pretty low to me.