Answer: darkness
In "The Negro Speaks of Rivers," Hughes talks about the experiences of black people. He recounts the places where they have existed, and shows that there has always been a river: the Euphrates, the Congo, the Nile, the Mississippi. All these experiences have made the black people what they are, something which he identifies as "ancient, dusky and deep." "Dusky" is used in order to highlight the darkness of the rivers, which in turn shows their depth. The depth mirrors the complexity of the African experience.