bwaylvsong- You still seem to be suggesting that blonde hair can only be the product of one white parent and one black parent. Two black parents are capable of having blonde children. I am a Jamaican and there are entire regions in Jamaica where you will encounter naturally blonde, dark skinned people who have not had a white descendant for generations. While true this is a result of mixed ancestry, these people consider themselves to be black, not mixed. It is for that reason that I said black and mixed-race people could both be blonde.
I don't have any qualms with the idea that Glinda was written with a white blonde actress in mind. I only wanted to point out that being black does not necessarily preclude one from having naturally blond hair.