My argument is more or less as follows:

1. The act of agent A blackmailing agent B costs agent A more than not blackmailing agent B (at the very least A could use the time spent saying "if you don't do X then I will do Y" on something else).
2. If A is an always-blackmail-bot then A will continue to incur the costs of futilely blackmailing B (given that B does not give in to blackmail).
3. If the costs of blackmailing B (and/or following through with the threat) are not negative, then A should blackmail B (and/or follow through with the threat) regardless of B's position on blackmail. And by extension, agent B has no incentive to switch from his or her never-give-in strategy.
4. If A inspects B and determines that B will never give in to blackmail, then A will not waste resources blackmailing B.