Crowley gives the boys his blood while telling them it might not be his. He also specifically tells them not to trust him. Yet, there's no sign that the bone/blood weapon will even work until it does. No sign, at least on screen.

Sam *knows* that the vial contains Crowley's blood. How? He can smell it. All that demon blood drinking before and with Lucifer. It's changed Sam. Write the missing scene where Sam reveals this to Dean.