Green’s Dictionary of Slang

choco n.2

also chokker, chokko
[abbr. chocolate n.1 (1)]

(usu. middle/upper class) a derog. term for a black person.

[US]I.L. Allen Lang. of Ethnic Conflict 46: Color Allusions, Other than ‘Black’ and ‘Negro’: chocolate [chokker and chokko are possible variants].