Green’s Dictionary of Slang

(US) a black person.

1859		C.W. Tayleure Boy Martyrs of 1814 10: Don’t budge, blackberry! [HDAS].
1894		Fort Worth Gaz. (TX) 21 Aug. 2/5: Facetious darkies called it a ‘Blackberry Special’ that hove into this city yesterday [...] About one thousand three hundred of them [i.e. African Americans] [...] disembarked.
1986		S. King It (1987) 676: Calling him names for nigger he had never dreamed existed – nightfighter, Ubangi, spade, blackberry, junglebunny.