I'm afraid that phrase doesn't quite work in English.
You cannot blackmail someone who doesn't want to to be blackmailed -> You can blackmail someone if and only if they want to be blackmailed...?
Nobody
wants to be blackmailed unless they're getting off on some kind of BDSM aspect of it. Otherwise it's a strictly hostile interaction; nobody wants to be robbed or beaten. The negative can work in this case (You need to learn that blackmail only works on those who
don't want to be blackmailed) but only because the alternative assumption is not that a person would want to be blackmailed, but that they simply don't care. And, frankly, that wording is more natural. "You need to learn that blackmail only works on those who care about being blackmailed" sounds more in line with the assertion of dominance at play here. "You need to learn that blackmail only works on those who care about your blackmail" could work as well, and focuses more on her dismissing what dirt he has on her rather than claiming to not care about blackmail in general.
Anyways, I was reminded of your game earlier today:
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