Eugene Volokh has an interesting post on blackmail.  The quandary: 1) it is usually legal to publish something that is true even if embarassing, and 2) it is usually legal to ask someone to ask for money in exchange for a legal action. So why is blackmail (promising to publicize embarassing information unless money is paid) illegal?  The quandary, as Volokh notes, is ignored in practice, although there are interesting exceptions.