I was wondering why we sing 'four colly birds', when most people these days sing 'four calling birds', and discovered the word colly means black and so the song's about blackbirds.
And then I came across a couple of references to collie dogs, so naturally I couldn't resist having a look at those sites. It seems the breed might be named for the black-faced sheep they used to herd in Scotland, or maybe they are named because the dogs themselves were black.
Here's a quote from The Border Collie Museum:
Combe goes on the say that "some say that the collie, or colley in its [early] English spelling, got it's name from the breed of sheep it herded." She posits that the black-faced sheep may have been called "coaley" for black. This idea is also promoted by the often quoted reference in Chaucer's Nonnes Tale "Ran Colle our dogge...", the implication being that he was a black dog, and that's why his name was Colle (like "Blackie").
Merriam-Webster online dictionary seems to agree with Combe. Their entries for "colly" and "collie" seem to support the idea of a coal-black origin