I also had this problem with some of my companions, including Blackwall, on the PC. When this happens one of the things I try is changing my HUD options. I almost always have it set to 'contextual' but when the person is invisible and other things don't work (walking around, saving and reloading . . .) I go into the Options menu and and change the HUD setting to always on. I've also found that sometimes not just walking around but turning around is necessary. Something about changing the camera angle makes them visible again. Good luck.