A recent comment in "The New Yorker" questioned whether "blackmail", as we normally understand it, should really be a crime. He used the example of the TV producer who has been charged with extorting money from David Letterman, threatening to expose his affairs. Apparently what the threat consisted of was that he was going to make a TV movie about Letterman's affairs, unless Letterman paid him two million dollars.
The conceptual problem here is that blackmail makes it a crime to threaten to do something which you can legally do. If you can legally do it, then how in the world is threatening to do it a crime? Interpreted broadly enough, many acts which people do regularly may be perceived as "blackmail". But it should not be criminalized.
Wikipedia defines it thus: "Blackmail is the crime of threatening to reveal substantially true information about a person to the public, a family member, or associates unless a demand made upon the victim is met. This information is usually of an embarrassing and/or socially damaging nature. As the information is substantially true, the act of revealing the information may not be criminal in its own right nor amount to a civil law defamation; the crime is making demands in exchange for withholding it. English Law creates a much broader definition of blackmail, covering any unwarranted demands with menaces, whether involving revealing information or not."
Blackmail is to be distinguished from "extortion", which consists of threatening to do something criminal to the person if money is not paid. This clearly is, and should be, a crime.
I am reminded of the Kansas attorney who nobody liked and the bar was anxious to get rid of. He was disbarred for sending threatening letters to people prior to suing them, demanding money. The thing about this is that it is common, and the better practice, to send a demand letter prior to suit. This gives the defendant a chance to engage in negotiations toward settlement if he is so inclined. To sue somebody out of the blue, without prior notice, is certainly not good practice. And yet, the attorney was disbarred for this, showing just how far this nebulous concept of "blackmail" can be extended if you stretch it.