The blackjack duels was not something I expected. Definitely a different approach.
Mechanically, though, it seemed like the best route was to Stand the moment there was risk of busting, since the punishment for busting not only took away your chance to hit, but also gave the enemy a huge free hit. The computers also approached the game a similar way, and at the end of the day made the final boss feel a bit like it was pure chance if you could win it (even though I got it first try).
I also feel it would be a boon to pull cards from a deck for both you and the opponent, and give them suits. It would make the blackjack aspect feel a little more there, and maybe add some strategy in the way of card counting.