A criminal act of extortion, malicious threatening to do injury to another to compel him to do an act against his will. Usually involves the threat to release information, often true, about the person that will defame his reputation or bring criminal actions against him.
The criminality lies not in the release of the information
at least if true - but in the extortionate aspects of the threat to do so.
When rents were reserved payable in work, grain, and the like, they were called reditus nigri, or black mail, to distinguish them from white rents or blanch farms, or such as were paid in money.