Here’s one of the crazy pseudo scientific theories, seems like it is only “popular” in Japan
How exactly does blood type personality theory work? According to Nomis book, the ABO blood type determines personality, temperament, and compatibility with other people. Specific blood types are characterized by representative traits that can be used to distinguish between different individuals.
While use of blood type personality theory seems harmless enough, there is definitely a darker side. A new term, burahara, has arisen in recent years to describe the harassment that those with the “wrong” blood type may experience (burahara is derived from the Japanese words for “blood” and “harassment”). Since Type B individuals are seen as unpredictably and unreliable, employers have refused to hire qualified applicants with the “wrong” blood type. Information on blood type remains a standard question in job interviews despite efforts by Japan’s Health, Welfare, and Labor Ministry to curb the practice. Children in some kindergartens are grouped according to blood type and cases of childhood bullying relating to blood type have been reported. During the 2008 Beijing Olympics, Japan’s women’s softball team had their training schedule adjusted according to the blood type of each member of the team (the fact that they won a gold medal did little to discourage blood-type enthusiasts).
Despite efforts by Asian psychologists to educate the public about the “sham science” surrounding blood-type personality theory, its popularity and the continuing sale of books with titles such as “Instruction Manual for People with Type B Blood” means that the fad will likely continue. At least until the next fad kicks in.
More info on the Wikipedia Article: Blood types in Japanese culture.