[Facts] Re: Why are the Black Sheep Turks thus named?
According to Brewer's Dictionary of Phrase and Fable, these two different tribes of Turkomens got their names from the standards they carried into battle.
The Black Sheep Turks came first, so my own guess is that their adversaries devised a standard for themselves which depicted the exact opposite image -- that of a white sheep -- to further emphasize their opposition to the Black Sheep Turks.
-- Nanaea