When used idiomatically, probably an allusion to a police officer blowing his or her whistle on observing a violation of the law.
blow the whistle (third-person singular simple present blows the whistle, present participle blowing the whistle, simple past blew the whistle, past participle blown the whistle)
- (idiomatic, usually with “on”) To disclose information to the public or to appropriate authorities concerning the illegal or socially harmful actions of a person or group, especially a corporation or government agency. In some jurisdictions, it is illegal to fire a person for blowing the whistle on an employer.
- (team sports) To make a piercing sound which signals a referee’s action or the end of a game.
- (disclose information): tattle, tell on; See also Thesaurus: rat out