This "negro in overalls" is the second driver in The Sound and the Fury: he agrees to drive Jason from Mottson back to Jefferson for four dollars (313).
In The Town this "delivery boy from Christian's drugstore" regularly brings "his ritual tray of four coca colas" for bank employees at the end of the business day (323). The novel does not specify his race, but typically in the Yoknapatawpha fictions delivery boys are black.
In The Mansion Mink sees "a few Negroes" shopping - "trafficking" is the word the narrative uses - in "the small dingy store" in Memphis where he buys animal crackers (319).
In The Town, these Negroes are regular customers at Garraway's store on Seminary Hill. Gavin Stevens describes them as "loafing" (327), and he and Mr Garraway mutter so as "not to be overheard: two white men discussing in a store full of Negroes a white woman's adultery" (329).
In The Mansion the "young Negro man" whom Mink sees inside the small store in Lake Cormorant is wearing the "remnants of an army uniform" (286). He obeys the store proprietor's command to drive Mink down the road, but at the same time subtly tries to let Mink know that the white man had cheated him.