A person who reports or informs on a wrongdoer within a corporation, government agency, etc.
An employee who publicly alleges wrongdoing at his or her company or who reports this information to appropriate government agencies.
We owe it to a small group of brave whistle-blowers that we know about the infractions at all.
Probably an allusion
to a police officer blowing his or her whistle on observing a violation of the law.