Some levels are normal, some are low. "Skin color" affects production as you pointed out, but that has literally nothing to do with what we are discussing.
For knowing how much blood level is "good", if you go back [1], that's why I suggested a good starting point would be looking at any native with an average pre civilization type days worth of outdoor activity and attire. For example, native African in their sunny climate, native European in their cloudy climate, etc. You probably also have to look at their pre-civilization diets, if they were too different from the average today (like traditional Inuit diet).
And, as I said, if there's no difference to be expected in baseline "healthy" vitamin D levels across genetic populations (a native African or native European), you could just go same some people in Africa or Papa New Guinea, right now, where you can find some populations living (at least very close to) pre-civilization lives, and measure theirs for some reasonable ground truth.
Cheers!