It would seem to make it impossible to commit to blackmail when the action of blackmail has negative utility. How can you possibly convince your rational future self to carry out a commitment they know will not work?

You put the answer in the title. We are humans, not rational agents. We have built in mechanisms to handle this. Pride, embrace it. This actually becomes easier with experience. I've found that in times when I've tried to be a good little CDT agent and suppress my human instincts it has gone badly for me. My personal psychology doesn't react well to the suppression and I've actually been surprised how often failing to follow through with a threat (or what should be an implied threat) had more negative consequences than I anticipated. On this my instincts and my ethics are aligned.