I am B+. So is my mom and she doesn't have to UC (but does have diverticulitus). I don't buy into it either. Our ancestors ate what was surrounding them in order to survive and now we are advanced enough now to eat what we want.
I'm pretty positive there is no British in me. I'm mostly Swedish>German>Irish>Polish.
I personally don't think that there is one common factor among all UCers that led them to get the disease. The body is far too complex for something so simple.
EDIT: I was looking on the site that suebear posted and it says Blood Type B are those who can thrive on dairy products. I've been lactose intolerant for a long time. Even before my UC.