I would think that the reason black women in this country would have low vitamin D levels is because of their pigment. Their skin is darker to resist the sun in the tropical areas and when in northern climes not enough vitamin D gets through.
Different races have adapted over centuries for the climate in which they live and races living in areas to which they are not adapted fare less well.
Detroit has a lot of clouds and a lot of people with dark skin, and a very high incidence of rickets.
Also around 43 degrees North latitude.
I’ve read that the darker your skin, the less vitamin D you produce from sunlight.