I have brown hair.
It has a categorized as higher levels of the dark pigment eumelanin and lower levels of the pale pigment pheomelanin
I wonder why brown or darker color hairs are more common than blond or red hair.
blond" is first documented in English in 1481 and derives from Old French "blund, blont", meaning "a colour midway between golden 
So records show that blond hair didn't exist before? Do you think it's possible for new hair colors to naturally start existing then?
Yes. Do you know that People with brown hair are often referred to as brunette, 
Yes, I wonder if there's a name for people with black hair since we have brunettes, redheads, and blonds.
There may be.  in French is the feminine form of "brunet," the diminutive of "brun" (brown, brown-haired or dark-haired).