I don't know. The switched at birth thing seems viable. It's also not impossible that somewhere back in their ancestry, a Brit or a Roman or whoever that conquered their native area got into the mix somehow.
But there's also THIS possibility. What if, and I know I'm reaching here, but, what if this is because of the "souls" that are coming here now? The creation of the blond haired blue eyed body is needed for certain souls is it not? At least that's MY understanding of it. And THOSE "souls", those people are needed here at the time they are able to be of use?
Just some thoughts to throw out there.