21 Blitz: Single Player Black is a free-to-play card game developed by Tether Studios LLC for Android devices. It is a unique combination of Blackjack and Solitaire, making it an exciting challenge for aspiring card counters. The game is designed to be played in a few minutes, but players can spend hours setting high scores.

The gameplay is simple; each turn, players choose one of the four lanes to play a card and make card stacks that add up to 21 to score points. Going over 21 will bust the lane, and three busts will result in the end of the game. The game offers various bonuses, such as streaks and lane bonuses, to help maximize the score. Players can also use the "UNDO" button to peek ahead at the next card or reset any busts.

The game provides smooth touch controls and fast loading times, making it an enjoyable experience for players. It also includes a "How to Play" guide to help players learn tips and tricks. There are alternate game modes that introduce new challenges, and players can customize their card backs and background art. Overall, 21 Blitz: Single Player Black is a fun and addictive game that delivers a unique twist on classic card games.