A blackjack is a short, thick stick that is used as a weapon. [US, informal] Police confiscated guns, knives and blackjacks. regional note: in BRIT, use truncheon.
But the idea is that when someone shouts "monkey" they are not really asking for the dealer to bust, they are asking for the next card to be a "10" card which may well bust the dealer if he has a stiff of course.