The various nilotic groups of South sudan, southern Ethiopia and eastern Chad generally have the darkest skin colours among Africans - groups like the Dinka, Nuer, and Anyuak have some of the darkest, smoothest, almost totally flawless skin pigmentation in Africa. Some Senegalese groups also have very dark skin tones, though generally not as dark as the Sudanese.
I was trying to think of the Dinka tribe, before I was accused of racism.
I'm sorry you find discussing pigmentation so difficult.