Green’s Dictionary of Slang

(US black) a token black employee, hired to parade the liberal racial attitudes of a white-owned organization.

1876		Times-Picayune (New Orleans) 20 Dec. 2/5: He should have got a nomination for Congress [but] he was cheated out of it by that little showcase nigger Antoine.
1967		Star Gaz. (Elmira, NY) 24 Sept. 52/1: In all these films he has been a showcase nigger.
1969		Springfield News-Leader (MO) 9 Oct. 54/2: Integration seemed to lead only to invisibility [...] he had become a little more in the eyes of many whites than a genteel nigger — a showcase coon.
1971		Sydney Morn. Herald 28 Feb. 63/6: When he did ‘Paris Blues’ with Paul Newman [...] he was accused of being a ‘showcase nigger’.
1982		Yazoo Herald (MS) 16 Jan. 1/2: We’re not talking about ‘showcase niggers’ [...] those blacks [...] who have titles but no responsibilities.
1995		William Missouri Downs Innocent Thoughts 🌐 Hell. For a pay check I do what I got to do. (In Black English) If they is a black jury, suddenly I’m a brother, man. (In White English) If they are white, I am a perfect showcase nigger.
2003		P. Monaco Sixties 150: In such roles, Mason concluded crassly, [Sydney] Poitier was serving simply as Hollywood's ‘showcase nigger’.