Perhaps I am missing something, this seems to be an accurate model of opportunism of which blackmail itself is a subset. Namely, both examples given are a type of opportunism which the agent exploits the target's desire for a particular value (either reputation or a MacGuffin) for profit, despite the risks of lost value balanced against the target's anticipated value. Blackmail, colloquially, is typically used to denote the above by use of incriminating information (such as steamy letters), while purchasing the MacGuffin may be called any number of things depending on the character of the act.
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.