The koreans may have been the ones who started the movie 'My Boyfriend is Type B', but the beliefs towards blood type in relation to one's character originates from the Japanese. I heard its even normal for new friends in Japan to ask each other about their blood type. Nah they are not vampires or anywhere near that but over there, blood type is believed to tell alot about a person's character, just like the western or chinese horoscopes. According to the general beliefs, the blood types are classified this way:
Type A - calm, composed, level-headed, serious, reliable, trustworthy. Appearing aloof or superior, they are often surrounded by people of their own type, as they find it difficult to rely on other types. Most Japanese are type A.
Type B - cheerful, enthusiatic, excitable, self-involved, capricious. They always appear bright and full of energy, but they are often hiding an inner lack of confidence.
Type O - carefree, generous, popular, strong-willed, adaptable to new things. They are the diplomats, who maintain group harmony but occasionally make big mistakes by not being careful enough. Most Americans are Type O.
Type AB - sentimental, considerate, cautious. There are two sides to AB types: their behaviour is significantly less direct with "outside" people than with "inside" people who they are comfortable with.
Unsurprisingly, science does not support any of these beliefs. An individual's personality is formed by a complex mixture of genetic and environmental influences, and while blood group is genetically determined, any influence on personality must necessarily be very small, if it exists at all. Having said that, i still strongly believe in western horoscopes. The difference is quite big actually - blood types are divided into 4 different types only and there are so many people so how can many people have similiar personalities? Whilst for horoscopes, there are so many different signs and each sign is further broken down into male/female which makes it more realistic and accurate isn't it? Just imagine, if there are 50 people of type A living under one block would it mean all 50 of them are similair to one another in terms of personalities? That doesn't makes sense.
In case ur wondering, i'm type O and i don't think i'm a diplomat but the rest of the characteristics a Type O person is said to possess are all kind true on me.