Bob has been accused of ripping off the melody from a very old song that originated in Canada by slaves who escaped from the U.S. The song is "No More Auction Block". There is a version by Dylan on the Bootleg Series 1-3. I think I have a pretty good ear, but if his version is even close to the original they are pretty diiferent melodies. There is a tiny guitar bit after the chorus that has a similarity to the chorus of Blowin', but that's the only thing I heard that is even close.

No More Auction Block is a very powerful song by the way. Those in the know seem to think that he learned it from Odetta.