Hair color is one of the most distinguishing aspects of a female's appearance. If you ask men, most will have a preference as to which they think is most attractive.
Personally, I think that brunettes are the most attractive granted other factors such as skin tone play a part, too. For me this relates to experience. The brunette girls I know are simply more fun. They enjoy doing outdoor activities, sports, going to a movie, among a bunch of other things. So technically it is not the hair color itself, but what personality I associate with each hair color.
Hair color is not a main criterion of attractiveness for me. A girl's complete physical appearance and her personality are much more important.
"Going back to Marilyn Monroe, and maybe even further, there has been this portrayal of blondes as extremely attractive women"
While this does not apply to me, there appears to be a general consensus that men prefer blondes. A lot of this has to do with the perception and portrayal of blondes.
Everyone has heard the saying "dumb blonde." This notion may appeal to certain guys because they think if they get a blonde girl they can get her to do more of what they want. Yes, this is a pathetic notion, but many stereotypes are. Also there is the saying: "blondes have more fun," and although likely false, the saying still exists. Furthermore, in the media there is the portrayal of the blonde bombshell. So the notion of blondes being the most desired women type is everywhere.
"A girl's complete physical appearance and her personality are much more important than just the hair color."
Going back to Marilyn Monroe, and maybe even further, there has been this portrayal of blondes as extremely attractive women. There is a desire to look like Barbie, which entails a small waist, large breasts, and perhaps most importantly long blonde hair. Due to the perception and portrayal of blondes, there is a belief that they are preferred by men. Whether this is true or not remains a question, but the belief is certainly out there. And in this context, I am a non-believer.