To expose something to the public, often something scandalous or deceptive. That company's stock price plummeted after the media blew the lid off the CEO's embezzlement scandal. An anonymous tip to the police is what blew the lid off the theft ring.

To blow the lid off something means to publicly expose a scandal or illegal activity; to reveal the truth about something; to let the public know about a big secret. “He threatened to blow the lid off the company’s tax evasion schemes.”