It is a widespread belief in Japan that character is linked to blood type, as this BBC Magazine story details:
According to popular belief in Japan, type As are sensitive perfectionists and good team players, but over-anxious. Type Os are curious and generous but stubborn. ABs are arty but mysterious and unpredictable, and type Bs are cheerful but eccentric, individualistic and selfish.
About 40% of the Japanese population is type A and 30% are type O, whilst only 20% are type B, with AB accounting for the remaining 10%.
This seems crazy:
A whole industry of customised products has also sprung up, with soft drinks, chewing gum, bath salts and even condoms catering for different blood groups on sale.
Seems to me like you can revel in or excuse your personality traits on your blood type. When it is working for you, great. But when something bad happens, blame your blood type.