There is no definitive answer to this question as the legality of online blackjack can vary from state to state. In general, however, online blackjack is generally considered legal in the US as long as it is played within the confines of state and federal law.
In some states, such as Nevada, online blackjack is considered a form of gambling and is therefore subject to certain regulations and taxes. However, in other states, such as New Jersey, online blackjack is considered a form of skill-based entertainment and is not subject to any specific regulations.
Ultimately, the legality of online blackjack will largely depend on the jurisdiction in which you reside. If you are unsure about the legality of online blackjack in your state, it is best to consult with a legal professional.
