blackjack has 12 distinct synonyms spread across 5 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a common scrubby deciduous tree of central and southeastern United States having dark bark and broad three-lobed (club-shaped) leaves; tends to form dense thickets
a flag usually bearing a white skull and crossbones on a black background; indicates a pirate ship
a gambling game using cards; the object is to hold cards having a higher count than those dealt to the banker up to but not exceeding 21
Synonyms, antonyms and the broader and narrower relations for blackjack come from Princeton University’s WordNet. Full sources and licences.