The Joker card has no function or standing in the game Blackjack. However, in a home game, it could be used as the cut  card. That's a card that is slipped into the deck where one wishes to cut , or separate the cards without revealing the bottom card inadvertently.

Blackjack (formerly black jack and vingt-un) is a casino banking game. It is the most widely played casino banking game in the world. It uses decks of 52 cards and descends from a global family of casino banking games known as "twenty-one".