Graymail

Graymail is the threatened revelation of state secrets in order to manipulate legal proceedings. It is distinct from blackmail, which may include threats of revelation against, and manipulation of, any private individual. Graymail is used as a defense tactic, forcing the government to drop a case to avoid revealing national secrets.

Graymail can occur in two ways:

In the United States, the Classified Information Procedures Act of 1980, also known as the Graymail Law, was designed to counter the second tactic above by allowing judges to review classified material in secret, so that the prosecution can proceed without fear of publicly disclosing sensitive intelligence.

...
Wikipedia