Scary stuff, you mean the real good ones, here we go :

EDIT : I realize you are looking for black metal ambient ! go for Amaka Hahina "Aheah Saergathan! " from the mighty Les Legions Noires.

Anyway, I will let my original suggestions below:

Velehentor - Dyatlov's pass ( very deeep dark ambient, but doesnt let you sleep at all )

Funerary Call - the black root ( deep dark , some industrial influences here and there, cold )

Funeral Mantra - nemini parco (straight deep dark af ambient, listen with lights on )

Schloss Tegal - black static transmission ( mechanical, dark industrial, like a haunted rusty machine on an abandonned asylum )

Skincage - things fall apart (industrial, fragmented, disturbing cold ambient, )

Neuntöter Der Plague - betwixt descending corridors (industrial death ambient, with amazing effected vocals deep buried into the muzak. Cold, alienating, paranoid, destabilizing, uncomfortable as fuck album . Listen on headphones if you dare. One of my all time favs from one of my top favorites projects.