Blood type doesn't fall into the category of dominant/recessive genes exactly; rather it combines this with the properties of incomplete dominance. Ignoring the Rh factor, there are 3 alleles for blood type, I^a,I^b, and i. You will be type A if you have I^a I^a or I^a i and type B if you have I^b I^b or I^b i. You can also get type AB by having the combination I^a I^b or be type O if you have i i. If you need to use dominant/recessive, you can say the A and B allele are dominant over the O allele and codominant with one another.
Answer:
They have homologous structures because they have a common ancestor.
