I've now discovered the brightness keyboard hotkeys! So at least I don't have to fiddle around in settings if the screen goes dark. You will need to disable Automatically adjust brightness in System. I also need to do a bit more detective work on the circumstances when it happens and will try unsetting Slightly Dim The Display on Battery Power to begin with, as the battery life is so good I don't think I really need this. I have a newer 2013+ Intel GPU on the Mac, and fullscreen video has blue artifacts. If you don’t see the Automatically adjust brightness checkbox, you can manually adjust the brightness. I'm not thinking of returning my Macbook at this point as I'm assuming it is far more likely to be a software issue that will eventually be fixed in an OS upgrade rather than a hardware problem. If your Mac has an ambient light sensor, choose Apple menu > System Preferences, then click Displays. I have Automatically Adjust Brightness and True Tone enabled, as well as Slightly Dim The Display on Battery Power. I use Firefox as my browser and certainly for the two occasions today that was open. Once again I used Display in Settings to increase the brightness. It's just happened twice today whilst using it int the kitchen, for no apparent reason. I managed to get Display up in the settings and with some difficulty used the slider increase the brightness. The first time I used it outside in daylight (not super bright sunshine) it dimmed to such an extent I could hardly see the screen content.

I've also seen this with my Macbook Air M1 running (now) 11.2.2.

Turn off auto adjust brightness mac