Green’s Dictionary of Slang

black jack n.2

[SE black + generic use of proper name]

the ace of spades.

[UK]Kipling ‘Black Jack’ in Soldiers Three (1907) 103: Black jack, Sor, I shud expaytiate to you, is th Ace av Shpades which from time immimorial has been intimately connect wid battle, murdher, and suddin death.