There is a strange problem when using MCE Blaster with my HP LP2465 monitor from 2006.
If I use CGA or EGA modes, when the border is not black, there are problems with the brightness of the main part of the screen.

Black/Blue border

But if I push the adjust button when the border is active (so the image will be misaligned), there is no such problem.

It's clearly visible that the text's color changes when the border color changes

However, my Samsung S22F350FHI from 2017 does not suffer from this problem.

It appears that the border falls on the blanking intervals, so when it's not black, it produces a signal where it shouldn't be.
Modern monitor tolerates it, but older not.