In Japan, your blood type is kind of like your zodiac sign. Theres little personality characteristics associated with each type. Its common knowledge in Japan to know your own type. When I was living there, I was asked several times by teen girls what mine was and by the look on their faces when I told them I had no idea, you would think they had just asked my name or something.

http://en.wikipedia.org/wiki/Japanese_blood_type_theory_of_personality