Answer:
People often overlook the fact that, logically, blondes should be more intelligent than any other hair colour, particularly this and the previous generation (although this is merely an average. Exceptions DO apply). The evidence is a) most men look for an attractive female spouse, which, in years gone by, was generally a blonde, b) most women like a successful husband, who, consequently, should be intelligent, c) intelligent parents can teach their children to do well in school, and d) intelligence (or, rather, learning ability), hair colour (in this case, blonde) and appearance are passed down via DNA.
So, in answer to your question, they aren't actually. For 'dumb blonde' to be passed down through the generations, there would have had to have been completely stereotypical 'dumb blondes'. Before the stereotype came into common usage, around the early 20th Century. Since no blonde would be pressured into appearing this way for the benefit of the men who traded in their dignity to sell films and books before then, if a blonde descendant genuinely happened to be less intelligent than most, it would not be the fault of the genetics not the hair color. There are limitless reasons on why that could have been caused, but it is unlikely to be genetics, due to said reasons above.