Honestly I feel the settings have been altered here, I took the following picture of my deck with my phone some months ago to indicate a bug stating I have been banned from vegasVR when trying to play it on the steamdeck, not actually banned at all, it loads shortly after but not this is beside the point - https://imgur.com/VXPGBVk
Those blacks look pretty black to me, in general day to day I can tell the screen is not OLED black but its not Game Gear black as you are showing above.
Honestly I feel the settings have been altered here, I took the following picture of my deck with my phone some months ago to indicate a bug stating I have been banned from vegasVR when trying to play it on the steamdeck, not actually banned at all, it loads shortly after but not this is beside the point - https://imgur.com/VXPGBVk
Those blacks look pretty black to me, in general day to day I can tell the screen is not OLED black but its not Game Gear black as you are showing above.