As I got more and more involved in manga I began to notice that in the character profile pages blood types were mentioned. The first time I saw it I thought the author was just being funny/cute about some irrelevant fact about the character. But soon I began noticing it in all my series. Although I raised an eyebrow at that random fact I never really bothered to look up why it was mentioned. I just thought that blood type was a physical characteristic that the Japanese liked to share and nothing more.