As near as I can tell Japan is the only country that has this myth. When I was an English teacher I told my students not to ask that question, unless they're a nurse or a vampire. They shrugged and decided there must be a special taboo among Westerners against discussing blood types!
It evolved in Japan just because people are so damn SAME in Japan. We have myths about redheads being aggressive, blondes sexy, etc.... well, in Japan everyone has black hair, everyone has brown eyes. So they needed something else to feed this strange human desire to categorize people's personalities by physical characteristics. So they hit on blood type.
I love telling people in Japan that I don't know my blood type. I do, but this way is more fun. They blink and look at me as if I'd just told them I wasn't sure whether I was male or female.
