Green’s Dictionary of Slang

1. (US black) a very dark-skinned person.

1947		M.H. Boulware Jive and Sl.
1983		I.L. Allen Lang. of Ethnic Conflict 45: ‘Black’ and Terms Modified by It: [...] black-out.

2. (S.Afr./US) black coffee.

1941		J. Smiley Hash House Lingo 5: There is still considerable toleration of originality in tagging servings judged by give it to hitler, blackout for two.
1944		P. Kendall Dict. Service Sl. n.p.: pass the black out . . . general issue coffee.
1960, 1975		Wentworth & Flexner DAS.

3. regulation knickers.

1943

Chicago Trib. 28 Aug. 8/5: If [she] happens to be a WAAF, she is wearing blackouts, twilights or passion killers, all names for her uniform knickers.

4. (US) a restaurant or store taken over and controlled by a group of (young) African-Americans (who then leave without paying for the food or goods).

1992

Maledicta Monitor 🌐 blackout a restaurant or store taken over and controlled by a group of (young) Negroes [who leave without paying for the food or goods]. Used by restaurant managers.