User blog comment:IamJakuhoRaikoben/Chapter 499: Gray and Juvia, Review./@comment-4945830-20160831204621/@comment-7418318-20160831220555

Maybe Juvia is a universal doner (type 0) or Gray is a universal recipient (type AB). If either of that is the case it doesn't matter what type the other one has.

Of course there's also the Rhesus Factor, but that's a 50/50 chance.

All in all FT has beaten far faaaaaaaaaaaar worse odds than that. And as Alpha Olphion pointed out, magic might also be able to overcome such "minor inconveniences" like incompatible blood types. ^^