Black has some sexual connotations. As a color it feels mysterious and dangerous, which can be exciting and seductive.
Black hair is also very common hair color in both anime and reality. It is probably the most flexible of all colors since there are few hard characteristics that define it. Audiences have few expectations when it comes to this hair color archetype. Black hair sometimes has natural blue highlights. Because of this, many anime characters use dark blue colors instead of pure black.
Kagome Higurashi from Inuyasha is a classic example. She is a normal high school girl who happens to have latent spiritual powers. She has the strongest powers in the series; however, she is unable to control them. She has the resiliency of a gold haired character and a temper like red head. She is strong willed yet vulnerable at times.
Another example is Roy Mustang from Fullmetal Alchemist. Roy is strong willed and intelligent. He has a sense of humor. Roy seeks to become Furor and wields strong alchemical powers in the show. Generally, he is calm and calculating.
There are many examples of black haired characters. This is one of the few color archetypes that has little to no expectations associated with it. Using black hair is a good choice for an author wants to have a character stand on its own actions and not use audience expectations to sketch the personality. Black hair characters can take longer to establish a sense of personality since there are few stereotypes to work with.
