What is the meaningful difference between the two scenarios?
Within the boundaries of the formal structure you've given the difference between blackmail and acceptable negotiation practice is largely a matter of social norms (and the usual complicated mess of framing games, preferences and status). We have no rule that people must sell or buy at the worst possible price that is still positive to them (in my culture at least, there are exceptions). We do have a (justifiable) instinct that revealing shameful secrets about people is an act of aggression and we are inclined to consider aggressive acts made conditional on something else that is not itself aggression to be 'blackmail' or 'extortion'.
That description is supported in my mind somewhat by the observation that people consider things to be blackmail/extortion/threats even when they are mere descriptions of a Best Alternative to Negotiated Agreement. To my best approximation, for most people the evaluation of whether X is bad dominates the formal structure, rather than merely resolving between formally equivalent cases.
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.