The Blood Mountain album probably has the most varied vocals Mastodon has done. They actually have two vocalists in the band, though I think the bassist handles most of them. On previous albums, there wasn't as much variety in the vocals. I just recently purchased an older album by them called Remission and the basic vocal style was basically hardcore vocals. Leviathan introduced some clean vocals (Seabeast, Naked Burn, Heart's Alive) as well as a different hardcore/gutteral vocals that were actually comprehesible or clear sounding (Blood and Thunder, I am Ahab, Iron Tusk) and then some screechy sounding vocals (Island, Aqua Dementia, parts of Heart's Alive).
But on Blood Mountain, they use a lot more stoner rock/metal vocals (Colony of Birchman, Hand of Stone, This Mortal Soil, Sleeping Giant), some Mudvayne-esque aggressive vocals (The Wolf is Loose, Capilliarian Crest, parts of Circle of Cysquatch). You get two different sounds from each vocallist, in previous albums I could tell which one was singing but the new vocals sound different enough that I can't distinguish who is singing what.
I would have to say that in each song there's basically at least of couple of moments where they use aggressive vocals. Even the one instrumental, Bladecatcher, has this schizophrenic gargling noise for vocals in it. Pendolous Skin, the other instrumental, is the only exception as the vocals sound like the singer is moaning unintellible words.
For me personally, I listen to Mastodon for the musicianship rather than the vocals, but I really enjoy the vocals on Blood Mountain. So that easily makes this my favourite album by Mastodon, but I'll have to give Remission a few more listens just to make sure.
Wow, that was quite a mouthful...
