I have OLED tv as my main monitor, and adjusting the settings of the screen allowed me to achieve this using a black image at full screen. The screen looks like it is turned off. I even tested with a dark room. No light comes from the screen. Once you get that set, then using black in UI colors should achieve this if I understand what you are asking.

I’m not familiar with HDR capablity within Linux though, and many newer OLED screens have some of these capabilities. That could be another element to consider depending on your screen capabilities and settings available.