
This card is exceptional as your team gets bigger.
In a typical 3-4 player game, monsters will be engaged to your far friends more often than to you. Now, you could spend an action to Engage, or invest in a Riot Whistle. But blackjack is cheaper, straightforward, inexhaustible, keeps your accessory slot free, and its upgrade isn't bad either.
When you run up to a friend on the brink of defeat, try to whack their worst nightmare on the head, and then see a tentacles, there's nothing more relieving than knowing that they're still going to live because you used your Blackjack.
So, if you're not using both of those hand slots, why not fill your offhand with a blackjack? I had a great Leo Anderson campaign in Carcosa running Blackjack & Survival Knife with a party of 4 investigators.