Although it is standard practice around here, "blackmail" is a weird name for this phenomenon. Why not "extortion"?
In the broader world, "blackmail" is a particular kind of extortion in which the threat is to reveal information (occasionally people use it more broadly, particularly in "emotional blackmail," which seems to just come from the same kind of sloppiness that led this community to call it "blackmail"). Calling bargaining "blackmail" sounds weird because bargaining has nothing to do with revealing information, but it is quite common to recognize a fine line between bargaining and extortion.
On topic:
Presumably the distinction between extortion and trade is in counterfactuals: in the extortion case the target is better off if they are known to be unwilling to respond to extortion, while in the trade case the target is better off if they are known to be willing to respond to trade. That seems like the most promising way to get at a formal distinction. Certainly this is tricky and we don't yet have a good enough understanding of decision theory to make clean statements. But I'm not sure if thinking about it as being about a "default" is useful though, we can take a step forward by replacing default with "what would happen if the trader/extorter didn't think that trade/extortion was possible." This makes it clear that norms are mostly relevant insofar as they bear on this counterfactual.
It makes sense that an extorter would like to convince the target of extortion that things would have been much worse for the target if they weren't expected to play ball, in exactly the same way that the extorter would like to convince the target that things will be much worse for the target if they don't actually play ball, or that a trader who wants to sell X would like to convince people that X is more valuable.
This kind of deception seems pretty straightforward compared to the more sticky issue of trying to make logically prior commitments, and the general fact that we don't understand very well how decision theory should work.
Do you expect that decision theory will reach a single conclusion here, or that it'll be a more scissor-paper-stone situations (hence the ideal outcome is to minimise the loss to extorsion and maximise the gain to trade, while accepting they're in tension)?
To illustrate an old point - that it's hard to distinguish between extortion and trade negotiations - here's a schematic diagram of extortion, alternating actions by player B (blackmailer/extorter//blue) and V (victim//violet):
The extorter can let the default Def happen, or can instead do a threat, ending up in point T. Then the victim can resist (Res) or surrender (Sur). If the victim resists, the extorter has the option of carrying out their threat (C) or not doing so (¬C).
You need a few conditions to make this into a extortion situation:
A mere threat doesn't make this into extortion. Indeed, trade negotiations are a series of repeated threats from both sides, making offers with the implicit threat that they will walk away from the deal entirely if that offer is not accepted.
But if C is worse that Def for V, then this seems a true extortion: V will end up worse if they resist a extorter who carries out their threats, and they would have much preferred that the extorter not be able to make credible threats in the first place. And the only reason the extorter made the threats, was to force the victim to surrender.
Fairness and equity
Note that this is not about fairness or niceness. It's perfectly possible to extort someone into giving you fair treatment (depending on how you see the default point, many of the boycotts during the civil right movement would count as extortion).
The all important default
This model seems clear; so why is it so hard to identify extortion in real life? One key issue is disagreement over the default point. Consider the following situations:
These examples should be sufficient to illustrate the degree of the problem, and also show how defaults are often forged by implicit and explicit norms, so extortions are clearest cut when they also include norm violation, trust violation, or other dubious elements. behaviours. In a sense, picking the default is the important thing; picking exactly the right default is less important, since once the default is known, people can adjust their expectations and behaviour in consequence.