Interesting thread. All I can contribute is that a lot of southern English gypsies sang a song usually called 'Down by the Old Riverside' or variants of that title, which has all the Blackwaterside verses and more. There are various versions of it on the new Brazil family CD put out by Musical Traditons. See
http://www.mustrad.org.uk/records.htm
.
I was always intrigued by the double standards in the song - the boy promises to marry the girl, seduces her and then says he wouldn't marry someone who did such a thing. (Bastard!)

As to the origins, all I can say is that the English versions tend to be longer which suggests it travelled Westwards across the Irish Sea. But that's only a theory,

Tradsinger