Blonde hair in anime often is used for foreign characters or the rich. In anime, many Western characters are rich. Golden locks are considered happy and regenerative in nature. Golden characters are not the most energetic, but their abilities to recoup when knocked down make them resilient.
Blonde hair is often an object of desire. The color is the same as the most precious of metals and often represents a character (most often a girl) that is unattainable by the protagonist or audience. This, again, plays into the rich/noble persona.
Oddly enough, blonde is also a “dumb” color. Blonde characters can be airheads and naive despite the noble and desirable traits of the color. Dumb blondes are a Western stereotype that has leaked into Asian ideas of blondes. These characters can be goofy and even trivialized by other characters.
Fullmetal Alchemist is a series populated by gold characters. Considering it is set in a country similar to Germany, this makes sense. Edward Elric definitely has the naive and sometimes dumb traits of the hair color despite also being a genius; he also has the regenerative aspects of the color. Nothing gets him down for long. The blonde dumbness isn’t necessarily a measure of intelligence. Sometimes it is an impulsive and compulsive rush into action without thinking ahead. In the case of Fullmetal, the color is mostly used to establish the European ethnicity of the characters. Although most of the characters do illustrate the resilience of the color.
One of the most famous blondes is Naruto. He certainly has the naivete and goofiness. The hair color also sets him apart from his mostly traditional haired counterparts. This plays into the trivialization blondes often receive in anime. Naruto is the main protagonist, but at the same time he is trivialized by the other characters because of his personality and hijinks. He can only amount to nothing. Like any good blonde character, he shows the naysayers how wrong they are.
Death Note’s Misa is a good example of a blonde against the grain. She first appears to be the stereotypical dumb looker: naive, popular, rich, and unattainable. However, she quickly shows she is intelligent, cunning, and very much attainable for Light. She has more of the hyperactive energy of a red-head than the resiliency of a goldilocks. She also has the cruel nature reds can possess.
Hair color is just a tool some authors can use to quickly develop a character. In the case of Misa, it can be used to surprise the audience.