Um...You have been misinformed. I have known black people who had naturally blonde and red hair. I have know a couple that even had gray hair in middle or high school.
Why is it that black girls have the same hair color as there skin but white girls have different hair colors?
Its all in the genes.
To start with gene factor but brown girls will also have black hair for that matter yellow race will also have black hair color.
Genetics and Genetic mutations are responsible for our hair color.