People with Type A blood are called "Farmers". Thus, they are gentle, passionate, affectionate, and obedient to others. They are very protective of their loved ones, and are cautious and careful. Many A's suffer from stress, but in the end adapt to what is around them. They can also be very stubborn. A is believed to have been the blood type of the Agarian Age. Almost half of the U.S. populus has Type A blood.
How does this relate to our beloved Ami? Well, she is very gentle and compassionate, but does suffer from stress as we see in the S series. Her dream to be a doctor also can be influenced by her blood type. Would anyone want a doctor with type O blood, the Warrior? No, that sounds more like a politician to me. Ami, with her kind heart and soul, is an obvious carrier of Type A blood.