In blackjack, there are a few things that are considered illegal. The first is card counting. Card counting is when a player keeps track of the high and low cards that have been dealt, in order to predict what cards will be dealt next. This is considered illegal because it gives the player an unfair advantage over the casino. The second thing that is considered illegal in blackjack is card marking. Card marking is when a player marks the cards in some way, so that they can remember which cards have been dealt and which ones have not.
This is also considered illegal because it gives the player an unfair advantage over the casino. Finally, cheating is also considered illegal in blackjack. Cheating is when a player uses any type of fraudulent activity in order to win the game. This can include anything from hiding cards up their sleeve to switching cards with other players. Cheating is strictly prohibited and can result in being banned from the casino.