The Ava/Jake thing is something else that has me kind of "Bwuh?" over the blood thing. Neither of them drank any blood, they just stopped fighting it and BAM! Whole new world.

And Nick didn't need it. There was no indication that Dean would need angel blood to be Michael's vessel (though there was that split second after Cas slammed him against the wall and that would have been GUH).

So, again, he had seen that the blood wasn't necessary, he was TOLD the blood wasn't necessary, so how does the blood still "work" when he knows it doesn't make any difference? I think this is one thing we'll never know...