"The origins of the Black Orcs and how they came to be so different are shrouded in mystery. Some maintain that Chaos Dwarfs, growing tired of the unruly nature of their other greenskin slaves, magically bred the more disciplined Black Orcs. If this is true, then their experiment failed badly, for Black Orcs are willful and independent-minded, far more likely to lead than follow. As the tales go, the Black Orcs led a rebellion against their taskmasters and escpaed the shackles of slavery, heading for the mountains and freedom."

-Jeremy Vetock, 8th Edition Orcs & Goblins Army Book, pg 39