In Japan, a strong belief persists since the 1930's that a person's blood type affects their personality. This goes a little beyond inoffensive superstition, often functioning as criteria for employment or dating. Facts considered, it should go without saying that the Japanese value blood type way over, for instance, zodiac signs and other superstitions more common in the Western world, and since it is also a factor that seems to have some influence over Squaresoft's construction of characters (they do have a tendency to provide information on it), an analysis on Selphie's blood type and how her personality fits the description seemed like a logical thing to write.
Selphie is a type B. Leonardo DiCaprio, Jack Nicholson, Akira Kurosawa and Paul McCartney are some of the celebrities that share this trait. Type B's are described as extroverted, charming and optimistic; they are good at reading people, and will always offer help and support when needed. They are also characterized by their strong creativity and flexibility, usually being very goal-oriented and never leaving an issue unresolved, although they often act before they think. Depression won't be a problem: type B's don't dwell over things, instead adopting a laid-back, relaxed attitude in life.
A trait that is always mentioned when describing type B's is their individualism, or, at least, the strong will to do things their own way. Selphie can be like this on some occasions: for instance, when she insists part of the team should go to the Missile Base to try and prevent the launch, rather than leave it behind and simply warn Balamb Garden that danger was coming its way. Wildness and unpredictability are also common, as is the ability to adapt easily to new people and situations. Type B's have no trouble fitting in: they are unconventional individuals who love to connect and can't imagine life without touching and being touched
(don't be a pervert, now). An additional, funny detail that I came across was that they are likely to judge appearances (but never their own): several scenes involving Selphie immediately came to mind upon reading this, such as when she mentions how ugly Rinoa's self-made train model is, or the time she makes a sarcastic observation on the unattractiveness of the Forest Fox leader. While I don't think Selphie judges people based on their appearance, she certainly judges the appearance itself: not only the old woman's, as mentioned earlier, but also Laguna's and Squall's, for instance.
In conclusion, Selphie's blood type fits her personality and take on life surprinsingly well; if you're not too sure, just take the time to compare the description above with my thorough analysis of her character. On a side note, Zell's blood type is also B, which was a coherent decision of Squaresoft's, since he shares many of Selphie's more upbeat, energetic traits.