Why is the "default" special here? If bargaining or blackmail did happen, we know that "default" is impossible.

This seems like you're setting up a game scenario, and then telling us at the beginning of the second turn that we should ignore what the other player did on the first turn.

Initiating negotiations is an in-game choice. The blackmailer's choice imposes a cost on me, me pre-committing makes the blackmailer's payoff 0-0.

And in most cases it takes extra work to attempt blackmail. On the other hand, You should be open to blackmail by your enemies, since it would benefit them to harm you.