If one parent has the pure A blood type (both alleles are A) is with genotype AA The other parent has the pure B blood type (both alleles are B) is with genotype BB
Crossing these two: AB AB AB AB
which means that all of the children will have AB blood type (phenotype). This happens because of the codominance. The A and B alleles are codominant and therefore, if an A is inherited from one parent and a B from the other, the phenotype will be AB.