The long and the short of blackmail is that it consists of two acts, each of which, were they to occur alone, would be considered legal by everyone. Yet somehow, when these elements occur together, virtually all commentators who have ever written on the subject consider the complex act consisting of both elements to be unlawful. There is only a corporal's guard that demurs. Is the mainstream view due perhaps to some sort of alchemy? How else can two legal "rights" be rendered a "wrong" when they take place in tandem?
Let us consider the specifics. Which two acts together constitute blackmail? First, there is a threat or an offer, depending upon your point of view. Whatever it is called, it states that some act, which in and of itself is perfectly legal, will be done. The proposition typically is to engage in free speech rights and gossip about the secrets of the blackmailee or target. However, the topic could be almost anything. The proposition could be to build a fence on my own land that blocks your view. It could be to write a negative review of your recently published book. It could even be to withhold selling you a piece of my property.
Block, Walker and McGee, Robert W.
"Blackmail from A to Z: A Reply to Joseph Isenbergh's "Blackmail from A to C","
Mercer Law Review: Vol. 50:
2, Article 4.
Available at: https://digitalcommons.law.mercer.edu/jour_mlr/vol50/iss2/4