If my response to the situation you described is to precommit to whatever the blackmailer wants, that is what makes the blackmailer want to blackmail me in the first place. If every simulation of me shrugs and flips my blackmailer the bird, the blackmailer has no incentive to punish me. You can escape punishment if you are prepared to flip the bird and accept it before incentivizing it. There may be blackmailers that enforce their ultimatums whether or not you will respond to them, but in that case akrasia doesn’t help.
If my response to the situation you described is to precommit to whatever the blackmailer wants, that is what makes the blackmailer want to blackmail me in the first place. If every simulation of me shrugs and flips my blackmailer the bird, the blackmailer has no incentive to punish me. You can escape punishment if you are prepared to flip the bird and accept it before incentivizing it. There may be blackmailers that enforce their ultimatums whether or not you will respond to them, but in that case akrasia doesn’t help.