I'm with Greg on this one; that style of singing was simply a lot more common and widespread in the past, and the fact that it survives in only isolated areas of the UK today doesn't of itself prove anything; though I'd keep an open mind on the question pending more detailed research. It may be that the congruity of Highland Scottish and African liturgical singing may have reinforced the style in the New World, but it won't have been a simple "either or" thing.

I'm not sure how relevant it is to suggest that Berbers may have been darker in the past than they are now (or, really, what they have to do with it at all). They were still Northern Africans, not members of the nations from which slaves were mostly drawn. The issue is one of musical culture, surely, not colour.