I think "blow" may derive from "blown up" - exploded. This image is one of intense emotion. I just explain the slang "blown away" a while back - it means intense suprise and awe.

Hi, to blow one's mind is a slang term. It means to affect with intense emotion, such as amazement, excitement, or shock.In this case it would mean that you were totally amazed by something.

American English idiom: You blow my mind! This means amazed me more than anything else before in my life.