It has to do with pigment. So high levels of eumelanin makes the hair dark while lower levels of pheomelanin make the hair lighter.
I have brown hair, quite common I guess. Yes, it is below black hair for most common hair color. And what is the least common hair color? Red hair naturally happens in 1 to 2% of the human population although I'm not sure fi it is the least common hair color. I say 1 or 2% probably makes it the least likely. Do you know what makes hair dark like my brown?
It is characterized by higher levels of the dark pigment eumelanin and lower levels of the pale pigment pheomelanin.