Severus blocks them because, frankly, he's good at it, and because Sirius is so angry right now that everything he does is as obvious to him as an oncoming freight train. "Try again," he tells him, still sitting on the floor, one knee up, wand pointed at the other. At this rate they're going to have an audience. "You don't have to ask me if you want to get better at dark curses, I'm sure cousin Bella would love to offer you some pointers. Might help you fit in--"

In your cell in Azkaban probably never gets out.