Midrash su Geremia 38:12

Rabbi Tachanah said: The Israelites also are called Cushites, as it is said, "Are ye not as the children of the Cushites unto me, O children of Israel?" (Amos 9:7). Just as the body of this Cushite is different from all creatures, so do the Israelites differ from all the nations of the world in their ways and by their good deeds; therefore are they called Cushites. One Scripture saith, "And Ebedmelech, the Cushite, said" (Jer. 38:12). Was it Ebed? Was he not Baruch, son of Neriah? But just as this Cushite is different in his body from all other people, so was Baruch, son of Neriah, different in his deeds and good ways from the rest of the sons of men. Therefore was he called a Cushite.