This is a prevalent trope wherever blond hair occurs naturally in the population. (Where it does not, the color may signify foreignness.) Since hair tends to darken with age, blondness does correlate with youth and the innocence correlated with that. Fiction runs with this so that the women are Colour-Coded for Your Convenience.
Often contrasted with a dark-haired heroine — as the Betty in a Betty and Veronica, the Girl Next Door compared to the Femme Fatale, the Damsel in Distress rather than The Vamp, the Country Mouse instead of the City Mouse — or just lacking the brunette's Jade-Colored Glasses. A redhead may also contrast and serve as a rival, though she will likely be more action-oriented than the blonde. She tends to be the younger of the pair; this is even more likely to be true for the male version.
The blonde's youth may also make her more naive than her counterpart, which can, but does not have to, slide into the Dumb Blonde. On the other hand, she may regard studying and doing well in school as part of her responsibilities and so perform better than her dark-haired and irresponsible Foil.
When blondes are natural, blondness does correlate with youth and so is attractive. Women, therefore, dye their hair blond. But after a critical mass of blondes have dyed hair, it no longer correlates with youth. And it certainly doesn't correlate with innocence; the honest brunette who does not dye her hair, perhaps because she is not scheming to get a man, appears more innocent. Therefore, blond hair dye falls out of fashion and then blondes are once again mostly natural blondes and so the correlation recurs — restarting the cycle.
The trope generally presumes blond is the natural color, since the correlation with youth no longer holds once dye is used.
When the cycle is on Hair of Gold, lack of blond hair may convince a woman or girl that she is not beautiful — leading to Beautiful All Along.
Women with Hair of Gold are also prone to Innocent Blue Eyes or gray eyes (though this is less common in more recent times). This contains a certain amount of Truth in Television, but it is exaggerated in fiction. They also tend to have voices in the soprano range.
