Some excellent bands have been mentioned here so far. Some of my favourites (whether or not they are merely blasphemous or actually orientated around the occult) are:
Dissection, Inquisition, Archgoat, Blasphemy, Black Witchery, Beherit, Watain, Secrets of the Moon, Ofermod, Ondskapt, Necros Christos, Nehemah, Mortem (Peru), Deathspell Omega, Ancient Wisdom etc
Also, on another note, I've been enjoying quite a lot of dark ritual ambient kind of stuff lately. I've been listening to Zero Kama, Zoat Aon, Hexentanz, Enemite, Halo Manash, Okk-uLth, Funerary Call, Darkness Enshroud, Black Seas of Infinity, Aghast etc etc.
Anyone enjoy any of these bands and projects??