Why are blondes considered to be stupid by some people? What's the history behind this stereotype?

The blonde stereotype has changed dramatically over the centuries, and the latest incarnation is the "stupid" image. The short answer to your question would be that it's a combination of historical stereotype, bad press, and human nature. The long answer is more interesting.

Let's start with history. Blonde hair is a recessive trait, and therefore somewhat rarer than brown or black hair. That has traditionally made it desirable. A quick perusal of art verifies this. Browse a book of Renaissance art, and count the blondes. You'll see a good number of blondes, even coming from the Mediterranean, where most people have darker skin and hair. For that matter, check out all the reference to golden-haired Greek and Roman deities. The locals of both those areas tend to be beautifully olive-skinned and brunette, with blondes showing only very rarely.

Whenever someone has the good fortune to be favored by society purely because of how they were born, those not so favored are likely to resent them. Especially over time. By the mid-nineteenth century, blondes were considered seductive, conniving, sinful, and evil. Short stories and novels--and especially the three-volume wonders so enjoyed by young women of the time--often pitted a virtuous young brunette against a wicked, deceitful blonde. (The opposite of what we often see on television today!) This stereotype held for a few years, until bleaching came into vogue.

Suddenly, brunettes had the world of blondness opened to them, and, like magic, the evil blonde image vanished. Blondes were considered cute and innocent. (Remember all those pixie-like platinum blonde silent movie stars?) In those early days of hair dying, bleaching your hair blonde was much trickier than it is today. Ingredients were often expensive, and they are pretty volatile. Add too much peroxide, for example, and you risk burning your scalp (or ending up bald). Usually, the very-fair, bottle blondes, then, were the women who could afford it. Being a bottle-blonde marked you as, well, if not exactly wealthy, at least well-off enough that you could buy such luxuries. Blondeness became associated with wealth and social status.

By the mid-twentieth century, dyeing your hair was much cheaper, but the image of blondes as rare and desirable still held sway. Blondes (including bottle-blondes) were often seen as social climbers, cashing in on their wholesome attractiveness to join the socialite sect and raise their status. The party circuit became populated with fair-haired beauties. (Blondes have more fun, after all!) These party-girls often made their money and their reputations as trophy wives or by upgrading lovers. Rumors abounded, and blondes became seen as women who sleep their way to the top. The implication, then is that not only are blondes "loose," but their good looks are all they have available to them. They can't work their way to the top by virtue of their minds (and therefore, their minds aren't all that great). The fact that so many of the wealthy, happy-go-lucky uppercrust were blondes only added to the virulence of such stereotypes by adding an element of jealousy. All of this was cemented and spread by blonde jokes, which prey on these feelings to spread an image of blondes as stupid, vapid, and slutty. Certain famous (or infamous?) blonde celebrities have also contributed, most recently, dippy blondes like Jessica Simpson and Anna Nicole Smith.

There you have it. Blondes are seen as stupid because of reaction against the old image of blonde as beautiful, class jealousy, their image of being sexual promiscuous, and the contributions of the rich, famous, and airheaded.