Green’s Dictionary of Slang

(US) a black woman; later use also a black man.

1929		W. Winchell On Broadway 30 July [synd. col.] A ‘seal’ is Broadwayese for a colored gel.
1944		Mencken ‘Designations for Colored Folk’ in AS XIX:3 174: The DAE does not list such vulgar synonyms for Negro as [...] seal.
1978		W. Donaldson Balloons in Black Bag 90: If the Major calls him darky, [...] eightball, seal, jungle bunny, skunk or Zulu to his face, we’re going to be short of a valuable new comedy act.
1983		I.L. Allen Lang. of Ethnic Conflict 47: Color Allusions, Other than ‘Black’ and ‘Negro’: seal [often fem. 1930].