Say what you will about the presence of Sand Trolls or lack thereof... they're still doing worlds better than Dark Trolls...
http://www.wowwiki.com/Dark_troll
All over the place in Warcraft III. Strangely absent in World of Warcraft.
Which sucks, because I'd love to see some purple trolls.
And before someone says, "They exist in WoW as an offshoot of Forest trolls" yeah, well that's just something lore nerds cooked up to reconcile the fact that they went almost completely neglected in WoW. They were a distinctly purple sub-set of trolls in Warcraft III... not the mottled confusion they currently are through lack of further development in WoW.