Final solution is to use the back LCD only as a light blocker for the Blackest Black. Keyword is "scrot" in Linux. I have various solutions for Context Adaptive Brightness in Github. Depending on machine construction, they can adjust the back light Leds too, causing truly black screen, when %[mean] at "scrot" goes below certain level.
I know scrot as a screenshot terminal tool. How could it help here, or do you mean something else?