Precommitment should not be a feature of rational agents; I think that if we can define blackmail in a land of no precommitments, we have a pretty good definition of blackmail.

In the absence of precommitments, you can have options available that would play the same role.

This is a problem of dividing the gains from trade.

I haven't seen any theory that clearly divides what is division of gain from trade from what isn't. When two agents sit down with each other, and they both have the possibility of having built, or not having built, various objects that would have various positive or negative values for the other... Where does the blackmail or aggressive negotiations end, and the dividing begin?