However, it is against Roblox rules to implement any gambling or casino style games that involve real money in any way, including Robux, as Robux can be purchased for real money.
Blackjack and 21 are essentially the same game, with the objective being to reach a hand value of 21 or as close to it as possible without going over. In some cases, the term "21" may be used to refer to the game played in a casual setting, while "Blackjack" is often used in a casino context.