For this type of error, I generally have you perform one of two actions which almost always solve the problem:
First we'll reset the PRAM and NVRAM:
If that fixes it, let me know, otherwise we'll try the second option to change display settings.
Do not click ACCEPT until you validate that my answer solved your problem.
Okay, change the resolution back to 1024x786. Reboot, and what I'll do is Opt Out of this question so another Expert can help you. The resolution and PRAM are the only two things I've seen to solve a black border around a monitor.
As a side note, you won't be charged until you hit the Accept button, so when another Expert helps you solve your problem, you can select their answer.