I would need more information in regards to your system configuration:
Which version of Windows do you have?
Also, the exact processor model and the graphics controller:
On Windows 10
Click on the Windows icon, and search for "System"
Look for System Control Panel then note the processor model.
I have Windows 10.
Intel Core i3-4130 CPU 3.40GHz
Intel HD Graphics 4400
Have you tried selecting native screen resolution?
Does this issue happen in BIOS?
Did you connect a second monitor to see if the issue remains on the second screen?
I don't know what native screen resolution is and I don't know how to get to BIOS on windows 10. But switching to a different monitor got rid of the two black marks. So i guess I'll just have to get a new monitor! Thank you for the help, much appreciated!