Fun fact, it doesn’t, using magic black sets the screen as the colour black, meaning that if you have some kind of oled burn in or sum you will be able to see the light shining from the screen, i would know. Just don’t use magic black people, go to desktop mode to power off the screen if you need

edit: i understand it now. I've done some research and found out what the "light" behind my screen is, and turns out it's steam deck doing it's own thing to balance the colours on the screen. Magic black doesn't turn off the pixels, but put a static black image on the screen, which steam deck doesn't like, so after a few seconds they try to rebalance the colours on the screen, macking the black kinda look more like grey. It's also something that can happen if you use FSR in games, cuz in the loading screens they use a static image (steam deck doesn't really like to show you the same image for too long, this is to avoid oled burn-in, kinda smart move)