Personality traits, including scholastic aptitude, physical ability, learning styles, temperment, and cognitive ability is more varied within the sexes than between them. This means that there can be more distinctions between two individual women then there can be between a group of men and a group of women.
Likewise, personality traits, including aptitude, temperment, etc, are more varied within the races than between them.
Therfor, I have a hard time accepting that personality traits would not also have more variation within the four blood groups than between them.
Example: nearly my entire family is O. We're different than a family of Bs I know because we're all impatient as he** and tend to be melodramatic, wheras they are more pragmatic and self-restrained. However, if you compared my two brothers (both Os, both male, same parents and only 2 years apart) the list of differences would be quite long.