There wasn't any way Rita's book could prepare him for this sort of an answer. He thought the blastia were just an element used! Why was she acting so defensive about it? As such, when Rita yells at him, Strange just looks so damn confused.

"Not even for a moment? You'd obviously be supervising the magic. It's not like anything bad would happen."

Which Strange realizes is pretty much inviting something bad to happen, but still! There's no need to be so defensive!