Antonyms for the phrase "blew whistle on" could include terms like "stayed silent," "kept quiet," "kept secret," "protected," or "covered up." These phrases imply a sense of loyalty, discretion, or even complicity, as opposed to the act of exposing or reporting something that is improper or illegal. Other possible antonyms might be "aided and abetted," "collaborated with," or "conspired with," since they suggest an active involvement in wrongdoing rather than an attempt to stop it. Ultimately, the choice of antonym will depend on the context of the situation and the perspective of the speaker or writer.