If my pupils are half as determined as I am, they'll end up trying that magic themselves. [ It's stated in a matter of fact tone...though there's just a hint of resignation in Strange's voice. He's not entirely proud of the whole 'blood magic in the peninsula' thing but really, it's the least of his dubious decisions that he's made recently. ]

The least I can do is try to dissuade them from black magic by telling them of what happened when I cast the spell.