It's a cultural Japanese thing. Some of them feel that blood types can tell something about a person.

You could think of the way it works as a little like astrological signs and fortune telling.

Certain blood types are thought to make a relationship more compatible as well.

I have seen condoms with blood types printed on them.

Blood types in Japanese culture - wiki link.

EDIT: Admiral is right, it's just a superstition. And in North American terms it would kind of like knowing a porn girl's favorite topping.