There should be several sites out there where you could get info about bloodtyping. This is probably the worst. However we have, based on the sparse information we've collected formed our own theories and they fit pretty well. More importantly we understand our theory. Anyway here it is

The O blood type is the hardest to catagorize. I mean it has Aerith Gainsburough, the nicest character from Final Fantasy (except maybe Rosa), and Makoto Shi-shi-o, Mr. The Weak Die The Strong Survive. Not exactly alike. The one thing all O bloodtypes have in common though is they all have principles, and they are very stubborn about them. You better have a very good speech to convince them and even then... They also tend to be emotional. And they can be very, very arrogant. Ne Hiko-san?

Good Examples: Aerith Gainsburough
Makoto Shi-shi-o
Quatre R. Winner

A bloodtypes are very driven. They pursue their goals very seriously (see Heero Yue). They also are known for their loyalty although it may not be to the people you want them to be loyal to. Also as a note people with A blood types have been known to mask their emotions. They can do it very well, in fact sometimes so well they can even confuse themselves.

Good Examples: Sakura Kinamoto
Aoshi Shinamori

B bloodtypes are, as Kenshin put it, easy going. They take life a little less seriously then their counterparts. That's not to say that they are flighty or uncaring either. The best way to get an idea for this bloodtype is to see them in action. Cid Highwind from FFVII joins your team on a whim. Sano and Wufei are very laid back when something isn't irritating them (often). Just look over the names, recognise one and you'll get the idea.

Good Examples: Duo Maxwell-perfect fit
Selphie Tilmitt

AB bloodtypes are interesting. They all have the emotional mask that certain A people develop. They can't seem to show their emotions well. They may seem like total jerks (Yes, you Squall.) and they may look happy/indifferent all the time but they usually have deep feelings about the events around them. As Trowa said "I'm just like you; a man of emotions."

Examples: Seta Soujiro
Trowa Barton