Another theory is that it's easier for a man to tell whether a blonde is healthy or anemic than with a brunette. And since all men want to maximize the chances that their offspring will survive, they want a healthy woman.
Hence, the reason why brunettes dye their hair blonde is that their natural higher skin pigmentation they trick men into thinking they are healthy, no matter whether that is true or not.
After having the incomplete logic pointed out by mimewars I haste to add that this reaction from the male would not have anything to do with whether he wanted children or not, but rather something more of an instinct.