I can't say about elsewhere, but in Oz the genetics of most flocks bred for wool are generally computer controlled to within an inch of their lives. The only times you see black, brown or brindle sheep they're in those flocks (usually kept by hobby farmers) selling to the hand-spun yarn market. They certainly get rapidly culled from any flock aiming at 11 micron wool, as one dark fibre will reduce the price of the whole bale.

Cheers, Rowan