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.
One big difference I see is what counts as an "action"; arguably X={Publish the letters} "feels" more like an action than X={Reject any offer}. It seems that when what distinguishes a "blackmailish" offer from a "non-blackmailish" one is a discrete distinction (either you precommit to publish or you don't), making the "blackmailish" offer is "bad", but when there's a continuum between the most and least blackmailish versions (as is the case with the MacGuffin negotiation), the offer doesn't seem to be as much of a transgression.
I'm not totally happy with this tho, and to me the general problem of "negotiation in situations with no focal points" has no satisfying solution beyond racing to who precommits first.
That trick works against CDT agents, anyway. Against agents that implement a modern decision theory it leaves the negotiation problem approximately where it started. (This means that if you put ideal CDT agents and ideal TDT agents in a room with devices that offer similarly ideal pre-commitments and give them exchanges to make the CDT ends up accepting the worst possible positive trade.)