Blackjack is a minigame in Red Dead Redemption 2 (RDR2). It can be played in the town Rhodes, see the location below:
During some story chapters you can also play it in your gang’s camp but this won’t always be available.
The goal of Blackjack is to beat the dealer by having the total value of your hand of cards be closer to 21 than theirs without it going over ‘busting’. You are dealt two cards initially. Then you can request more cards to get as close to 21 as possible, but if you go over 21 you lose. A Blackjack is when you get exactly 21 points by adding up your card values. You can view the game rules in more depth in the game’s Help Menu > Activities > Blackjack. It involves a lot of luck though.
You must play every table game for 100% completion and for the trophy “Hobby Horse”. For all other minigames check out the Red Dead Redemption 2 Minigames Guide.