I rather lost interest in the winged sandal story, but for all the attempted complications, it remains quite clear. The Countess never owned it, and the Baron wants to secure his ownership first before offering it for sale. Whatever this is, it is not blackmail. Engrossing, forestalling, regrating, badgering, or cornering), perhaps, which aren't even illegal any more in English law.
A lot of law is about specifying exact rules. The difficulty of doing so, precisely enough to decide cases, does not imply that there is anything philosophically problematic.
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.