No one seems to have confirmed anything about the Spanish blood thing.

The "Gypsies" mentioned are probably the same ones as in Black Jack Davy and numerous other songs. When you look at a database under Gypsy you find many of those songs are listed as Child #200, and references are made to the other songs, in several incarnations here.

I don't understand how this fellow MacGuire is listed as the author of this song, which doesn't seem to be substantively different than the older and even the oldest forms of the song. He may have eliminated some verses, but many singers leave out verses.

Some of the best information comes from Lesley Nelson's Contemplator site