This card is a bit hard to quantify. The obvious comparison to it is against the Knife that is available to every other role in a neutral form. At the same cost and strength, there are only 2 factors to consider when deciding which one is better over the other.
Perhaps you don't consider blackjack or Knife, because you prefer to carry .45 and machete as your includes in your deck. For those types of decks, then this isn't even a consideration. But in several cases I have considered machete and Knife/blackjack as it is a considerable cost savings over the .45 and 80% of the time the machete does the job of the .45 anyway.
Also, if you are solo, blackjack isn't for you. In large groups, you may be required to peel monstrosities off of people and help them out. In these situations, with the blackjack you expect to get some action efficiency.
1 health enemy (blackjack) - Action 1 swing 1 health enemy (Knife) - Action 1 swing or Action 1 Engage, Action 2 swing
2 health enemy (blackjack) - Action 1 swing, Action 2 swing 2 health enemy (Knife) - Action 1 sacrifice swing or Action 1 swing, Action 2 swing or Engage, Action 2 swing, Action 3 swing (sacrificing if needed on the second swing if the first missed.
There might be a scenario or two there missed, but that covers the majority of what you are looking for in comparison. The main situation where blackjack is best is obvious in the 1 health enemy category because the blackjack can save you an action and it doesn't make sense to sacrifice the Knife when it is on someone (in most circumstances).
If you don't care what happens to your allies, "swing away" and choose the Knife so you have an option for a burst damage opportunity.
If you do care about your friends (how nice of you), then the majority of the time you are using this weapon you are saving an action for fighting enemies for your allies. That was my thought pattern with using this for a four player campaign. On average over three scenarios so far, this has saved me perhaps 1 action per game. Also, if you wiff on your strong attack with the Knife you are left without a weapon anymore and the blackjack doesn't tempt you with that.
What I do like about the Knife though is that you could save an action with a single attack doing 2 damage when you need. So, this can also save you 1 action per game. Also, when you have that machete out in a situation such as this, the Knife can possibly be useful still for taking out 1 of 2 enemies engaged with you to turn that machete back online.
Is that worth having over the Knife? I tend to think that it is not in practice. The Knife just seems to be a better choice from my perspective. Just in case someone was going through these considerations as well, I thought I would share the experience I have had with the card here.