To the fact that Squall is a student at Balamb Garden, we are been given his bloodtype. This may be one of the reasons that Rinoa hasn't been given one, because she isn't a student. Also the matter of why Squall has a AB, rather then a A like most main characters would have, this concludes that the designers, think of which astrology, birthday month and bloodtype would fit with the character. AB has been given with care, looking at how the personality of many people are with this type AB, so Squall came closer to this one instead.

Type AB

Laid back and outgoing, shy and assertive ,a blend of opposites

unpredictable, compelling

may seem to have calm exterior -introvert and extrovert

friends find them dependable but they may rebel if they are overwhelmed and then withdraw

generous on their own terms

unpredictable in social situations 

shy with some, bold with others

strong creative strain-artist/performer 

everything they do is compelling

drawn to psychology, astrology, fortune telling, spiritual, blood

good politician or diplomat

could be too flexible

good at spotting problem and averting it

likes city but may feel claustrophobic

home has elements that inspire thought and action

hermit and social, subdued and assertive

doer thinker- torn in many directions- successful at what they do tasty with fresh vegetables

Opinion

Most of the aspects with AB, gives us a better view of how Squall acts and thinks. Laid back and outgoing rather isn't really what Squall is, but shy he is. As noticed I give my opinion a 100% good points on this type, either then that we can't choose for another one if their is a official Bloodtype.

Shy and assertive

Squall has occassionaly difficulties with expressing his feelings or what to say. This is mostly stated and seen with conversations between him and Rinoa. It seems that later he becomes more intertwined with his own feelings towards others, mostly towards Rinoa self. Being assertive is sometimes stated in the game, he knows what he wants, only he sometimes never knows what he has to do. Everybody seems to have a different point of view, not seeing what his own view may be.

Unpredictable

Squall may have been unpredictable, but Rinoa stated at a later time in the game that he is predictable afterall if you get to know him. People seem to like Squall more after a period of time, because he seem to act the same on occations. Like Quistis and Rinoa stating out ...whatever and ...none of your bussines at the same time Squall may say it. This seems if you don't know Squall, you may call him unpredictable, but later you can know how he acts. Rinoa uses this expression on most of the ways on their conversations, to lett him say other things, so he becomes more open.

Calm exterior

Squall seems calm on most occassions, not caring much about the fact what's happening. Other then that many people seem to scream at him and tell him to do something, this makes Squall confused and unsure about the matter, yet he stay's calm and goes to do what they have told him to do. This is mostly seen at the time Squall jumped of from the ladder and the other told him how scary it was. Out of the choices, Squall always acts calm even when the last one seems rational, he answers with a soft, yet calm expression.

Shy with some, bold with others

Squall may be shy with his feelings towards the people he really cares for, mostly if they are close to Rinoa. He is always unsure about what he should say and closes down. If it are people he doesn't know, he changes back into his lonely figure, not wanting everyone to know how he really is. This is just Squall's normal reflection, he yet doesn't know he does it, it normal for him to act with strangers on a bold kind of way, not knowing or wanting to know how they are, afraid he might care for them and loose them aswell. Squall got a great friendship with his teammates and trust them, because they grew up together. This is yet something different then coming closer to strangers.

Good at spotting problems

This is a solution that goes with Squall's personality. I can't say for sure that Squall is good at spotting problems, only he has a feeling to it when there is one. Like Rinoa he seems to solve problems down, sorting them out and understanding it. If he sees a problem, he think that it could endanger the mission, so he figures out by changing the course of action and completing it. When Edea seemed to shield herself form Irvine's bullet, Squall reassures Irvine he did ok, only he had to come in between to complete the mission with care. Not just because he is appointed for nor is the leader, he knows this is the path he has to choose.

Related

After some investigations, I have able to find that Laguna Loire does have the A bloodtype, which Squall could not have since they are related. Since Squall has the AB type, which is a negative setting, and his father the B type, it is clear that Squall's mother, Raine, would have been a A type. This makes clear that Squall has a family we know.