The Japanese have a fascination with blood-type that I would liken to the Western concept of astrological signs. The idea began when some western scientists proposed that Asian races were more closely related to animals because the predominant blood type in Asia is B, which is the blood type found most often in anmials. This, premise is both scientifically flawed and downright insulting (hello, Soclial Darwinism!) but as a result the Japanese scholar Takeji Furukawa published a series of articles in 1927 called The Study of Temperment Through Blood Type. Your blood type is said to have an effect on your personality- the Rh factor plays no importance.
Locke is type O, along with the Figaro brothers and Strago. In other Final Fantasy games, Aeris is also type O, and I'd give you an FFVIII parallel if there were any. In general, O is the most common blood type in the world, although as mentioned, B is predominant in Asia. O is also the universal donator- which is quite fittig for Locke and his hero complex and Aeris for obvious reasons. The Figaro brothers, too, are princes and leaders- and that is a sort of self-sacrifice. Figures like Cloud and Squall, by contrast, are type AB- the universal acceptor, something that encompasses all traits in a complicated whirl of personality.
Traditionally, Type Os are outgoing, creative, and popular. They enjoy being the center of attention and seem very confident in their action. They are also the initiator bloodtype, though they do not always finish everything they start. They are risk-takers, optimists, leaders, take-charge personalities. They see the big picture and not the little details clouding it. They are said to have an intrinsic elegance and a way with words.
It's quite easy to see why Locke is idenitified as type O- he has that red-blooded and passionate quality that leads him down stubborn and quixotic roads. He chases the adventure and savors the saving, and I suppose he has some great vision, not of the world- but of a life.