She wasn't a leader, but Shinfel Blightsworn was a Blood Elf Warlock who fought Cho'gall alongside the Horde.
Aside from her, we only have the Archaeology trainer, Elynara, who is - for all intensive purposes - the only real known Blood Elf Warlock in Silvermoon.
People need to separate the Sunfury Blood Elves from the Silvermoon Blood Elves. The former were the ones who did the whole "Warlock/Fel" thing and took it to the extremes. These are the "angry bad boy" blood elves that @
ravenmoon likes.
Silvermoon Blood Elves took up the practice of draining the magic out of living things...a questionable Arcane magical practice, not related to fel. In the main, Horde Blood Elves of Quel'Thalas, still remained as "Mages." Very few became Warlocks.