I wonder whether the meaning's got any connection with the Black Pig Dyke that separates Ulster from the rest of Ireland? because there's always been movement between lowland scotland and northern ireland?
a little something here http://www.scotchirish.net/The%20Plantation%20of%20Ulster.php4
and I guess you've seen stuff about the scary tailless/short-tailed/'cutty' black sow / Hwch Ddu Gwta.
but then again, the bit of the ballad here (which no doubt you've seen) http://www.borderreivers.co.uk[...]Villages/Cheswick/Cheswick.htm
is pretty innocuous, so maybe it was just about a Nice black sow. And no doubt pigs would be a bit more obvious thing to sing about in days gone by.