Is it possible for a human being to have black hair or can it only be very, very dark brown ?

Natural Hair Colors
A person's genes determines the hair-, skin- and eye color since before his or her birth. The determinants for hair color are known as melanocytes. These cells form the natural hair colors, or pigments and distinguish between black, brown, and red. Depending on the percentage of each pigment, the person has blonde, brown, red or black hair. As the melanocytes decay during a lifetime, the hair becomes a dull gray and later on it turns into a whitish color.

https://www.loc.gov/rr/scitech/mysteries/grayhair.html