How did the term "black sheep" originate ?

The Phrase Finder:

"Shepherds used to dislike black sheep as they aren't as valuable as white ones."


The Straight Dope:

"Black sheep, in those balmy pre-industrial days, were not as valuable as white sheep. None of the sources [Brewer's, the OED, and a few dictionaries of slang phrases] was explicit, but I presume white wool could be dyed into any color while black wool was more limited. Thus, the black sheep was the unwelcome oddity in the flock."

It goes on about the symbolism of "black."