black jack, n.⁴ — Green’s Dictionary of Slang

black jack n.⁴

(US gay) a black man’s penis.

1964		‘R. Blake’ Amer. Dict. Sexual Terms 18: black jack (Vulg Sl) A Negro’s penis, or any penis immensus, from the assumption that the members of that race are more penially endowed than others [Simes:DLSS].
1966		Trimble 5000 Adult Sex Words and Phrases.
1972		(ref. to 1940s) B. Rodgers Queens’ Vernacular 48: the penis [...] black jack (’40s).
1986–7		Maledicta IX 52: black jack n [R] Black man’s penis; homosexual slang.
1988		H. Max Gay (S)language.