I think the episode was about Cas learning (re-learning?) how to be a good angel - something all the angels seem to have lost along the way. Maybe not just this episode but his arc this season, TBH.

What did Crowley need/want Kevin's blood for? Is he addicted to human blood after the almost-cure he had? Or does he have something more nefarious up his sleeve?

Overall the episode wasn't as entertaining to me as the last couple had been, it lacked light-hearted moments. I think Dean is definitely feeling the weight of all the lies. :( I did like the Sam and Kevin interaction and really like seeing Kevin around the batcave.