The ABO blood type is controlled by a single gene with three alleles, rather than two. Each of us has only two sets of chromosomes, so any one individual has only two of these alleles at once. But the presence of three different alleles means that there are six possible genotypes, rather than the three possible for the more familiar two-allele situation.
For the ABO gene, the three alleles are the IA, IB and i alleles. We typically call these alleles "A," "B," and "O," but all three has to be represented by some version of the same symbol. In this case, that common symbol is the letter "I," which stands for "immunoglobin."
The i allele (the "O" allele) is recessive to both the IA and IB alleles (the "A" and "B" alleles). The IA and IB show co-dominance. This means that in an individual who is heterozygous for these two alleles, the phenotypes of both alleles are completely expressed, thus producing blood type AB.
Thus we have the following:
Blood Type A" actually means "blood cells tagged with antigen A." Antigen A is a specific protein marker found on the surfaces of all Type A blood cells. And the task of the IA allele is to cause the creation and attachment of this specific kind of antigen. The IB allele causes the creation and attachment of a different protein marker, the B antigen.
This explains why the IA and B alleles are co-dominant, and why the i allele is recessive. The IAIB genotype results in both A and B antigens on the cell surfaces. The i allele causes no antigen to be produced, and is thus a "silent" allele.