Blood types in Japan are considered to fit with certain personality types
just like astrological signs. This is only for O/ A/ B/ AB, not positive
(+) and negative (-).
From the rec.arts.manga glossary:
In Japanese pop culture, one's blood type is thought to be related to
one's personality. This belief became popular in the early '80s. Profiles
of manga artists or characters from manga will often include the blood
A very rough guide to blood
types:
A – nervous, introverted, honest, loyal
B – outgoing, optimistic, adventurous
AB – proud, diplomatic, discriminating
O – workaholic, insecure, emotional
Note: The characters' blood types are listed in the individual bios (supporting and senshi) and the basic information table.