Ulver's Bergtatt is a must for black metal. A lot of my favorite BM acts are avant garde. Emperor's "In the Nightside Eclipse" is a stellar album, along with Ulver's other two early albums, Kveldssanger and Nattens Madrigal (they moved into electronic music after that). Even those that have an obviously black metal spirit (by which I mean attuned to atmospheric and beauty, less concerned with rhythm, melody, catchiness) despite having little of the genre's trademark sound (Agalloch, Krallice, some of that odd Amesoeurs album).
And I highly, highly recommend you give Agalloch's The Mantle a try. Listen to the album in its entirety, and try to not let it fade into the background. Their music will speak for itself.