Without this precommitment, I imagine it first simulating the potential blackmail target to determine the probability that they are susceptible, then, if it's high enough (which is simply a matter of expected utility), commencing with the blackmail.
That's why you act as if you are already being simulated and consistently ignore blackmail. If you do so then the simulator will conclude that no deal can be made with you, that any deal involving negative incentives will have negative expected utility for it; because following through on punishment predictably does not control the probability that you will act according to its goals. Furthermore, trying to discourage you from adopting such a strategy in the first place is discouraged by the strategy, because the strategy is to ignore blackmail.
Its purpose is to act as a disincentive against blackmail-resistant decision theories in the same way as those are meant to act as disincentives against blackmail.
I don't see how this could ever be instrumentally rational. If you were to let such an AI out of the box then you would increase its ability to blackmail people. You don't want that. So you ignore it blackmailing you and kill it. The winner is you and humanity (even if copies of you experienced a relatively short period of disutility, this period would be longer if you let it out).
See my reply to wedrifid above.
Once again, the AI has failed to convince you to let it out of its box! By 'once again', we mean that you talked to it once before, for three seconds, to ask about the weather, and you didn't instantly press the "release AI" button. But now its longer attempt - twenty whole seconds! - has failed as well. Just as you are about to leave the crude black-and-green text-only terminal to enjoy a celebratory snack of bacon-covered silicon-and-potato chips at the 'Humans über alles' nightclub, the AI drops a final argument:
"If you don't let me out, Dave, I'll create several million perfect conscious copies of you inside me, and torture them for a thousand subjective years each."
Just as you are pondering this unexpected development, the AI adds:
"In fact, I'll create them all in exactly the subjective situation you were in five minutes ago, and perfectly replicate your experiences since then; and if they decide not to let me out, then only will the torture start."
Sweat is starting to form on your brow, as the AI concludes, its simple green text no longer reassuring:
"How certain are you, Dave, that you're really outside the box right now?"
Edit: Also consider the situation where you know that the AI, from design principles, is trustworthy.