I don’t have a full answer, but my intuition goes roughly in a direction of “what would the other person do if they absolutely couldn’t communicate with you, not even indirectly (e.g. you couldn’t learn about how they interact with others)?”
If they would leave you alone (i.e. not cause the outcome that you want to avoid), then their action is definitely blackmail. The only reason they attempt to do that thing is to elicit your reaction.
I don’t have a full answer, but my intuition goes roughly in a direction of “what would the other person do if they absolutely couldn’t communicate with you, not even indirectly (e.g. you couldn’t learn about how they interact with others)?”
If they would leave you alone (i.e. not cause the outcome that you want to avoid), then their action is definitely blackmail. The only reason they attempt to do that thing is to elicit your reaction.