The usual term is "blew the whistle". I've never heard "whistleblow" used as a verb.
It's not. "whistleblower" is a commonly used compound noun, and "whistle-blow" is starting to enter usage, but in common usage, it's "blow the whistle" and "blew the whistle."
Apparently the OP isn't the first to use it and it's kind of a predictable back-formation.
Both "whistleblew" and "whistleblown" seem awkward to me, though I am not enough of a grammarian to explain why (the latter sounds to me to more appropriate to the company than the person, in analogy to "wind-blown sand".) Maybe this is a case of impractical verbing^H^H^H^H^H^H^H formation of a verb from a noun.