Les you said some time back that You Could only find one reference to diguise and black face morris prior to 1750.
The wikepedia page on border morris seems to have found several references prior to that date both in Britain and Europe. The piece seems   be balanced as it refers to the minstrel theory as well.
Having read it I think it shows that the jury is still out on the matter.
No doubt supporters of both sides will read the article to support their preffered view.