I had to look up the difference between the two (blackmail and extortion), because I wasn't quite sure:
Extortion is a form of theft that occurs when an offender obtains money, property, or services from another person through coercion. To constitute coercion, the necessary act can be the threat of violence, destruction of property, or improper government action. Inaction of the testimony or the withholding of testimony in a legal action are also acts that constitute coercion.
Blackmail, in contrast to extortion, is when the offender threatens to reveal information about a victim or his family members that is potentially embarrassing, socially damaging, or incriminating unless a demand for money, property, or services is met. Even if the information is true or actually incriminating, you can still be charged with blackmail if you threaten to reveal it unless the victim meets your demand.