Sunbathing is not what made their skin darker, it was the melanin in their skin.
He is not asking why their skin is that color.
I think he really wants to know if there is any benefit to tanning if you already have dark skin.
Most people "tan" to change their skin color....heck...some pay big bucks to sit in a tanning bed.
I think he wants to know if staying in the sun has the same effect of changing the skin color of people who are already dark. Can they take the sun longer without burning? Does their pigmentation give them more protection? Would they have to "tan" much longer to notice a difference in skin color? Enjoying the beach and the warmth of the sun is a different thing.
This guy is asking curious things he has probably always wanted to know, but was afraid to ask.
It's the anonominity of the internet that makes it possible for him to finally do this.
When he finally satisfies his curiousity about black people, I expect he will next be asking curious questions about delicate female matters and sexual things.
Someone needs to tell him how to spell "g-o-o-g-l-e". Maybe suggest r-e-s-e-a-r-c-h. Some people honestly have no clue about how to find answers to their questions through research.