Sparky wondered and I googled!

Origins

It is not known clearly how this stereotype came to be, but seeing that Caucasian babies are often born with at least a touch of fair hair, and sometimes blond hair darkens as a person grows into an adult, blond hair could have come from that association of fair hair with childhood and youth, and therefore the person was seen as naïve and/or innocent and lacking the intelligence of an adult. Also, as blonde hair implies childishness, it can instill a desire to nurture among non-blondes and cause blondes to be the target of admiration. This may cause some blondes to behave in a childish manner, either unconciously or conciously, in order to gain attention and affection. Thus some blondes may be partly responsible for perpetuating the myth.

A possible origin is that dark hair and red hair have much more copper because of their coloring; copper was associated with intellect, and since fair hair has less copper due to its lightness, blondes were seen as less intelligent.

The stereotype could have its roots in ancient times; the ancient Greeks and Romans were fascinated by the fair hair of the Celts and the Nordic peoples and wished to emulate their red and flaxen tresses. People in the Mediterranean area often bleached their hair or bought wigs made from the hair of enslaved Germanic and Celtic peoples, and most notably the highest-ranking courtesans. Due to this association of red and fair hair with harlots, light hair earned a degree of contempt from the high-ranking ladies of society. Puritans, associating makeup and dyeing of hair with prostitution, forbade the dying or bleaching of hair, creating an imprint on dying hair that lasted until the 1920s. The "dumb" side could have been a way for wives of adulterous men to reassure themselves about the infidelity of their husbands; to think that their husband's blonde mistress was sleazy, worthless, ditzy, and not very intelligent[1].

Another possibility: the bleach used to lighten the hair in earlier times was powerful. If this bleach killed off brain cells it might have produced some genuinely "dumb blondes".

Yet again, the myth might stem from the idea amongst Romans and Greeks that Northern Europeans were barbarians and thus less advanced than Southern Europeans.

Or the "dumb blonde" stereotype could simply have come from modern potrayals, beginning very likely with Anita Loos's popular 1925 book Gentlemen Prefer Blondes. The story's central character, Lorelei Lee, is a beautiful but empty-headed singer. The roles that Marilyn Monroe, Judy Holliday, Jayne Mansfield, and Betty Hutton played certainly helped to spread the stereotype; all of these women played the "ditzy blonde bombshell" at some point, even though they were known privately as intelligent women. (Monroe even played the Lorelei Lee role in a musical version of Loos's book.)

Reality TV shows like Big Brother in the UK also don't help the stereotype. Since the series began in 2000, the show has hand 1 or 2 so-called "dumb blondes" as contestants on each series. Often the woman are hired for the show due to portraying themselves that way or are then edited to be seen that way. Almost all these woman are actually naturally brunette, such as Chantelle Houghton, Nikki Grahame and Helen Adams. Despite this, intelligent natural blondes like Shell Jubin who has an Art History degree and Vanessa Nimmo aswell as Grace Adams-Short have also been on the show.

Newspapers and magazines also play a big part in the blonde stereotype. Blonde jokes and labelling of blondes as "dumb" or "bimbos" are a regular thing in the media.

In modern day society, the blonde stereotypes are often used by scorned men who have been dumped or feel betrayed by a blonde woman or by brunette women who don't like the fact that blonde women are still very popular with the opposite sex and often considered more attractive than brunettes or redheads. This often leads to blonde women being berated or even physically attacked by other women. Nowadays, more blonde women are trying to conteract the blonde stereotype and take it as a serious insul