"But shouldn't the blame be on the actor, not the messenger? "

That would literally contradict everything you've said in your column.  By this new standard, neither Michael Flynn nor Donald Trump can be held responsible for sharing fake news.  Why?  Because they are just reporting the information they are given.  If journalists have no kind of obligation to fact check information before publishing, why on Earth would a non-journalist be expected to do that?  That makes no sense whatsoever.

So long as they aren't the ones who are originally producing the information, they can't be held responsible for sharing fake news.  That's essentially what you're saying here.  That's how you defend mainstream "journalists" who report fake news, by saying "well it's not really their fault that they were lied to."  But then why wouldn't you say the same thing about people like Michael Flynn?  Why should he be held to a higher standard than Washington Post journalists?