The real reason is that virtually all textbooks oversimplify the inheritance pattern for eye and hair color. Neither is due to a single gene with just two options. Look closely at the iris of almost anyone and you will see that there are many colors, not just blue or brown.

So, you might have 'blue' eyes, but in reality you could still be carrying alleles for brown, which can be passed on. For more details, you can check out

http://www.kumc.edu/gec/support/eyecolor.html