I agree. It was fleshed out better in the novels, especially after Jon Arryn's last words, which make sense if we consider blonde hair as a recessive tract into Baratheon House.

I still hope a lot of things were developped better, but I know I can't expect too much considering there are only 10 episodes, and a lot of things must be placed there.

However, I hope we'll have more sparks of "true Cercei" in the future ^^