Green’s Dictionary of Slang

(US) of people, black, African-American.

1943		I. Wolfert Tucker’s People (1944) 208: ‘You’re a known street-walker.’ ‘A real, regular mocha tart,’ said Egan.
1983		I.L. Allen Lang. of Ethnic Conflict 46: Color Allusions, Other than ‘Black’ and ‘Negro’: mocha [often fem. Probably from mocha, the color and the blend of coffee and chocolate. Used as an adjective since the 1850s].