Gray Bar in fullscreen mode multidisplay
There's a solution that I've found which should solve this issue with any application that you encounter this with. 


Download the free application Nocturne
Install, launch, and click the icon in the menubar
Select Preferences > then check Dim when inactive 
Adjust the slider all the way down when desired 
Enjoy! :)





  I've been using this application for ages, and without a doubt it's
  one of my favorites.