Im not sure if it'll help, but try adjusting the monitor from the controls on the monitor itself,. (Had similar issues when using a tv as a monitor) or tweak the screen size in the resolution in the settings. I know windows8.1 (at least on my os) auto detects the actual monitor size, so you might have to change the resolution to fit the screen.
I had the same problem. Go into the amd catalyst control center and check under My Digital Flat-Panels->Scaling Options, and move the bar if you can. I just checked in mine and its grayed out for some reason, but thats how I fixed the black border.