Green’s Dictionary of Slang

1. (US) rum sweetened with molasses.

1843		Merrie Men of Hoxton in Oxberry Budget of Plays I (1844) 105/1: He takes a pull at his black jack.
1863		‘Edmund Kirke’ My Southern Friends 112: A mug of ‘black jack’ helps him amazingly.
1877		Bartlett Dict. Americanisms (4th edn) 45: Black-Jack [...] 2. Rum sweetened with molasses. New England.

2. (US, also blackjack coffee) very strong black coffee, usu. sweetened with molasses.

1930		J. Tully Shadows of Men 61: On Sunday we got black-jack coffee.
1931		Irwin Amer. Tramp and Und. Sl. 29: Black Jack.— [...] the sailor and lumber jack have long so called the strong black coffee served aboard ships and in lumber camps.
1949		Monteleone Criminal Sl. (rev. edn).

3. (Aus.) treacle.

1932		Folkestone, Hythe...and Cheriton Herald 30 Apr. 6/5: The proud mother who presided introduced ‘Black Jack’ [...] contained in the form of a syrup in a tin.
1936		Courier-Mail (Brisbane) 10 Jan. 12/5: Some bakers, however, make white bread into ‘wholemeal’ by the simple expedient of colouring the white bread with ‘blackjack’ or crude molasses.
1945		Baker Aus. Lang. 81: Consider, for example, these slang names for treacle and golden syrup: longtail, spare boy, Kidman’s blood mixture, KIdman’s joy, beetle bait, black jack, bullocky’s joy, cocky’s joy and tear-arse.

4. (US) illegally distilled whisky.

1968

in DARE.