those comments weren't made to hate on blondes, ther were made to hate on women :p
this type of insult starting forming when blond hair became really popular-- so women would dye their hair blond and some people claimed that the chemicals would go to your head and make you stupid. it didn't help that the popular personality type, and favorable one of society, at the time was a proper, but rather stupid woman who is very girly. you know, the dress, the vacuum, the pot, pan, and apron.
Back then, it wasn't very hot to be very intelligent :p and what made you intelligent was listening to your husband.
so that insult just sort of.. grew to encompass all blondes and then to all people.You can accuse anyone of being a "blonde."
It's not that people hate blondes, because they don't.
But the stereotype of the "dumb blonde" exists for a reason. Some girls ARE naturally, uh, "less intelligent" and used to getting by on their looks, so they have never even tried to work on their brain much, and these girls tend to say a lot of dumb things (that are often funny). And then there are lots of girls who dye their hair blond, and take on that dumb personality because they think it's what guys want (and I suppose, some do). Even though they are actually smarter, they act dumb, and again, use their looks rather than their brains.
This became a common-enough occurance that eventually, a whole series of jokes have been made about it.
But don't take it personally. I've known a number of blond girls who were very smart and not at all afraid to let it show. They had no trouble getting respect.