Here's a few bands that are dark. Their correct genre is Stoner/Sludge Metal. Its really good and it makes your high really hit you.

IsIs http://www.myspace.com/sgnl05
Pelican http://www.myspace.com/pelican
Bongzilla http://www.myspace.com/bongzilla420
Mastodon http://www.myspace.com/mastodon
Jesu http://www.myspace.com/jesujesu

Myspace is being dumb, but heres a few more...
Baroness
Kylesa
Sun o)))
Constellation Orion

And with this site, you just scroll down and click the songs, its not metal, its very beautiful and heaven like music:
http://www.isaachurt.com/merch.html