Roleblocking takes priority over protecting. Meaning that if someone blocks a killer and another protects the killer's prey, at the latter it'll result that they didn't stop anything since the kill attempt never happened;
I thought really long and hard about how this would be relevant to anything. I settled on, it meaning that there is probably a watcher/tracker power. And it would be relevant if that tracker targetted the killer or the watcher targetted the killers prey. Alternatively we could have a baner who dies if they successfully stop a kill. So that's all probably worth noting and watching for.