Origin of words "Black Sheep"

Have you seen the telecom ad with the black sheep?

While the ad is a bit strange (why are black sheeps cheap), this did make me wonder about the origins of the phrase - black sheep and/or black sheep of the family.

Admittedly this task requires a trip to the library.

However, for now, I stuck to searching online. Miffed peevologists can embark on the trip themselves.

Online Etymology Dictionary explains that "real black sheep had wool that could not be dyed and was thus worthless".

So the meaning 'odd one out' comes down from this meaning?

While on the topic, I also looked up origins of the poem "Baa baa Black Sheep".

According to Doctor Bruno, the word baa might be used simply to associate sheep with sheep products. This in turn might have been due to importance of the wool industry.

Check out his blog for two possible historical significance evident in the poem.