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.

For instance, the blackmail could be "spend an evening talking over this with me, then I'll give you back the letters"

Still formal blackmail. If the blackmailer would incur a cost from publishing the letters, then the blackmailer would not bother in the world where the blackmailee simply ignores such threats.

"I own this bauble that can save your civilization, which will otherwise die. I precommit to rejecting any offer you make for this bauble that is less than 99.9% of the value of your civilization (so about 0.1% of your people will survive). You are, of course, at liberty to refuse.

This is a problem of dividing the gains from trade. In general we still haven't solved what is a good Schelling point for a fair division. Suppose a fair division would be to pay 50% of the value of your civilization, since the cost to the McGuffin-seller is negligible. Then you tell the McGuffin-seller that you've precommitted not to pay more than 10% of the value of your civilization, exhibiting changed source or something to make the precommitment credible. If the seller is a CDT maximizer, they say "Oh well" and sell at 10% which is the maximizing action from their perspective, since as a CDT agent they are ideologically committed not to take into account that they have caused themselves to be the target of this precommitment by being the sort of agent who would say "Oh well" and sell. It seems quite likely that if there is a 50%-of-value Schelling-point 'fair division' here then the rational action is not to accept any trade over, respectively under, 50% plus epsilon, respectively 50% minus epsilon. This may or may not end up being the same problem as formal blackmail in a completed theory, but it shares some of the same structure where you can exploit the living daylights out of CDT maximizers by moving logically first.