Sorry, I wasn't too clear :
The black bars are on the outside borders of all the screens as I scroll through, not just on one particular screen. For example, I have 4 screens and the border is in front of screen one and after screen four, but it shows up as part of the screen under the visible icons
Try setting the margins to NONE in your settings. It should be under Nova Settings > Desktop > Width margin/Height margin. Give that a try.