I see that you are no longer having a problem with Youtube on your V box.
I had the same problem as you were having with a black screen when i selected a video to watch. Here's the fix, In the app go to settings, select reset app. Follow instuctions to reload app, log back in with your google account and its up and running.
It's just apity that Virgin tech team don't seem to know about this, it would have saved 5 phone calls and about hours messing about rebooting the box etc.