Green’s Dictionary of Slang

(US black) a black penis.

1926		Blind Lemon Jefferson ‘That Black Snake Moan’ 🎵 Mmm, black snake crawlin’ in my room / And some pretty mama had better come and get this black snake soon.
1934		‘J.M. Hall’ Anecdota Americana 2nd Series 124: A Negro falls asleep [...] and wakes to find a blacksnake, which has crawled up his pants leg, putting its head out through his fly. ‘I knew you was long, and I knew you was black, but where’d you get them baby-blue eyes?’.
1960		(con. 1950-1960) R.A. Freeman Dict. Inmate Sl. (Walla Walla, WA) 11: Black snake – a Negro’s penis.
1968		N. Heard Howard Street 138: Your black snake’ll never crawl through this grass!
1995		(con. 1985–90) P. Bourjois In Search of Respect 38: They got their large mammoth [...] Alabama black snake with its magnets out plunging you. Alabama black snake found its way to the assets boy!
2000		Ebonics Primer at www.dolemite.com 🌐 blacksnake Definition: the penis of a Negro. From the common names for a species of long, black snake. Example: Why don’t you come suck my big Alabama blacksnake.