SOLVED (for me). I had this problem all of a sudden and i realized it had to do with my TV's display of my home phone's Caller ID. To remove the banding, i simply called my home phone number to reactive the gradient and it went away! Yay.
here's the deal...my Cox Contour TV displays a caller ID whenever I receive a call to my home phone number. When this happens the top edge of the screen darkens into a blueish gradient. After the call is answered or ignored the caller ID and gradient (should) automatically go away. At some point i noticed a call come in and I ignored the call, but the gradient did not go away. At first i didn't realize to consider this, I was trying to find some setting somewhere. Then i remembered seeing that Call ID notification and thought it must have just stuck in the mode.
I was right. I just called my home number, saw the Call ID displayed on the TV. Canceled the call. The gradient went away.
I hope this helps!