Of course it depends on the person, but Japanese, especially girls, are interested in personality profiling by blood group.
Most people tend to classify our personalities like below.
blood type A:punctual/attentive/nervous
blood type B:self-centered/moody/insensitive
blood type O:friendly/realistic/rough
blood type AB:smart/cool/split personality
As you can tell from the above, type B people tend to be considered they don't have strong points. Actually, we are not likely to like blood type B people.
I know it sounds very ridiculous to categorize people by their blood types, but I don't know why, we like it. Hence, we want to ask our new friends about their blood types, and almost all Japanese do correctly know our blood types.