We are often fascinated by horoscopes, with the notion that how stars align billions of miles away might say something about us in the everyday. In Japan the major personality index is much more biological in nature, you could even say "embodied" - blood.
Blood type holds a special place in Japanese culture as the go-to data for understanding what a person might be like. This recent article (or this more extensive one)
Indeed, anime and manga characters often have their blood types mentioned, and of course people keep track of this sort of thin in the form of online manga character blood type databases! Other websites, like this one called Anime Blood unpack the matter for readers.
Right now some of the top selling books in the country are on this very topic...
(the blood bag pictured here is by British designers Dunne and Raby)