Not sure how Boa fits the AB stereotype, but I didn't pay much attention to him during Evolution.
Japanese blood type theory, in an extreme nutshell:
A = analytical, organized, sensitive, easily stressed out.
B = emotional, creative, uncooperative, stubborn.
AB = Two-faced or having a split personality, creative, aloof, quiet.
O = confident, born leader, insensitive, outgoing.
(FYI: there's no documented evidence for any of this. It was popularized by a journalist in the 70's who had no medical/scientific background whatsoever. Even most Japanese know this is just fodder for women's magazines and anime character profiles).
"...I am a graveyard that the moon abhors..."