Basically, both of my parents are blood type A (both are confirmed and it's also certain both of them are my biological parents). I recently found out my blood type is AB. How is this possible? I tried reading about different mutations in blood type, but I couldn't find anything that was similar to my case.
For an individual, having blood group A, it is not necessary that he/she doesn't have the I^B allele. It may be possible that he/she has the allele, but allele can't be expressed on the surface of red blood cells because of the absence of the enzyme which creates the B antigen. Here, there can be a mutation which affects the enzyme. However, in the progeny, the parents pass on the B allele, and it is expressed in the offspring if the enzyme is functional in it. If an A allele is obtained from one parent and B allele from the other, the offspring has phenotype AB.
Reference- page 366 of iGenetics by Peter J. Russel, third edition.