For a more parable-ic version of this, see here.
Suppose I make a precommitment P to take action X unless you take action Y. Action X is not in my interest: I wouldn't do it if I knew you'd never take action Y. You would want me to not precommit to P.
Is this blackmail? Suppose we've been having a steamy affair together, and I have the letters to prove it. It would be bad for both of these if they were published. Then X={Publish the letters} and Y={You pay me money} is textbook blackmail.
But suppose I own a MacGuffin that you want (I value it at £9). If X={Reject any offer} and Y={You offer more than £10}, is this still blackmail? Formally, it looks the same.
What about if I bought the MacGuffin for £500 and you value it at £1000? This makes no difference to the formal structure of the scenario. Then my behaviour feels utterly reasonable, rather than vicious and blackmail-ly.
What is the meaningful difference between the two scenarios? I can't really formalise it.
I can't fit this to any of the examples you gave. The Baron comes to the Countess with a threat to publish their correspondence: it is clear to both that he has unilaterally introduced a change to the status quo, to the Countess' detriment. A McGuffin owner comes to a McGuffin collector offering a McGuffin at a fixed price: it is clear to both sides that this has introduced a new option, taking no existing options away. Everyone is in agreement. What situations are you thinking of that make "rare" the clarity of this distinction between threatening to injure and not threatening to injure?
And what about the variant when the winged sandal was going to be given to charity, but the Baron rushed in to prevent that, arriving just in time?
Here it's clear that the Baron still has legal ownership (just!), but that it's the Baron who's changing the status quo.
You could argue that a lot of law is about specifying what the disagreement point is (generally through ownership rules and contract law), but that doesn't mean that our legal system's choice of disagreement point comes from any intrinsic definition that makes sense (see the difficulty with intellectual property).