And, now you've discovered why Europeans have light skin [1]! And, why supplementation is suggested for those with dark skin [2] living in regions with light skinned natives!
[1] https://www.science.org/content/article/how-europeans-evolve...
> But the likely explanation for the pigmentation genes is to maximize vitamin D synthesis
[2] https://www.cooperinstitute.org/blog/african-americans-at-gr...
What does the color of skin have to do with the normal amount of a vitamin in blood?
Skin tone merely regulates how much you end up producing. Just because your skin is bright doesn't mean you need less of a vitamin.
I'm sure there is some variance in this, but I doubt it's swinging between 20 and 30 ng/mL
[1] also covers evolution of skin tone https://en.wikipedia.org/wiki/Vitamin_D
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.