I'm no expert on the subject of blood types and personality traits, but I'm sure that there are plenty of people writing about it online so I won't even try to launch into an explanation here. But the basic thought is that the blood type influences the personality of people (note that by no means do all Japanese people believe this, the ones who strongly believe in this are probably a pretty small but loud percentage of the population). But I think it's no understatement to say that basically all Japanese people know their own blood type without having to think about it.
Quite often I have been involved in conversations like this
Ms./Mr. A: Salaryman, which blood type are you?
Salaryman: Actually, I don't know
Ms./Mr. A: (shocked, in disbelief) But what will they do if you are in an accident and you need a blood transfusion?!
Salaryman: (jaded) You know, I think they will check in any case and not just take anyone's word for it so I'm not too worried about that...
Ms./Mr. A: (Confidently) Well, in any case, I'm sure you’re an A/B/O/AB!
Salaryman: (Amused) What makes you say that?
Ms./Mr. A: (Still confident) Because you have the personality of a A/B/O/AB
I never really figured out which types are the good ones and bad ones so I might not act properly offended when someone labels me as one with bad personality traits...