Are The blood proteins A and B codominant?

Yes, the blood proteins A and B are codominant.

In codominance, both alleles of a gene are expressed fully in the offspring, and neither allele is recessive. When individuals who are heterozygous for blood group A and B (genotype AB) produce offspring, both the A and B blood antigens are expressed on the surface of red blood cells. As a result, the offspring have the AB blood type, displaying both antigens.

This is in contrast to complete dominance, where one allele masks the expression of the other.