(SPOILER)I don't know why any character would notice or care. I certainly wouldn't think it any issue at all if blond/brunette couple in real life had blond kids. Assuming the brunette is a carrier, that's a 50/50 shot every time.
P.S. why is this a spoiler considering we've seen Jaime and Cersei smurfing like rabbits already? Admittedly there is a small step of logic between those deductions, but it is such a tiny step that even Tyrion's stride covers it 100 times over.