I have one of phvn177's modded GBAs.
When I took photos in the dark of his GBA and another modded GBA I got elsewhere, the black did look a bit lighter... but in person and under normal lighting, my eyes could not see a real difference in the contrast. In fact, his looks slightly more saturated and neutral than the other. I'm gonna say it has to do with screen wear, LCD calibration(?), or even where the screens came from. That last one assumes GBA screens were manufactured in batches from various sources and at various times, each with a slight difference in picture quality. That's pretty common with electronics.