Well, I just now checked and it was in fact Ron who first told Harry that (in chapter 11); he said his dad told him, but I imagine probably not till just that summer. Stan and Ernie mentioned Black had killed "a wizard" along with a dozen Muggles, so that part was apparently common knowledge but mayyyyybe the identity of the wizard wasn't. Or maybe I'm just talking out of my ass at this point. *rueful grin*