According to a study done by French researchers, people who viewed photos of blond women had a decreased intelligent performance. this was also true for women who participated the study.
The study was published in Journal of Experimental Social Psychology and they argue that stereo typing is the cause of this results. Having looked at someone who you assume is less intelligent than you are, you are more likely to act less intelligent.
This explains why in history, some heroes pretended that they were weaker, less capable, or less intelligent than their opponents under certain situations to get out of trouble. They understood the psychology and used it wisely.
Never underestimate someone by their appearance.
If you are interested, here is the link to the original paper