Doug, you're right that Obama isn't stupid enough to have offered Blagojevich anything but his appreciation for considering the replacement candidates he found acceptable.
But for him to have 'blown the whistle' on Blago, he'd have to have known what the governor was doing, and for all we know, it never got any further than Blago CONTEMPLATING "selling" the Senate seat and discussing with his own staff the wish to do so. We don't know that he actually ever proffered a quid-pro-quo to anyone.

The taped conversation where Blago says of Obama, "F___ him!" was not a conversation with a member of Obama's staff.    How do you know Rahm Emmanuel knew about that conversation?

It's also possible that someone on Obama's team did 'drop a dime' on the governor, but we don't really know if any of them had enough evidence of any illegal activity to warrant running to the feds with it.