>>My understanding is
that it is not being dark-skinned which predisposes to rickets.

Being dark-skinned is a risk factor for rickets.  Dark skin evolved as a
protection against the intense ultra-violet rays of the sun near the
Equator.  If you look at the original distribution of people before the
1500s (European exploration), you find that the closer you are to the
Equator, the darker skinned the population.  As you move north OR south from
the Equator, skin color gets lighter.  At the same latitude, savannah
populations are darker-skinned than forest populations because they have
more sun exposure.  The main problem with too much ultra-violet radiation
reaching the deep tissues of the body is not sunburn, nor skin cancer.  The
main problem is destruction of circulating folic acid, which then leads to
miscarriages and neural tube defects.  Regardless of how dark your skin is
genetically, you will get darker upon exposure to the sun.  This ability to
adjust the color of one's skin based on sun exposure and season of the year
is most noticeable in populations living in the middle latitudes between
Equator and poles, but is found in all populations.  In dark skinned
populations who live near the Equator, you don't see rickets, because sun
exposure is enough to produce plenty of Vitamin D, which is necessary for
the absorption of calcium from the diet, thus preventing rickets (calcium
deficiency).  In dark skinned populations who have moved far from the
Equator, where there is little ultra-violet radiation for much of the year,
and less intense UV radiation even during the summer months, such as
northern US states for example, being dark-skinned is indeed a risk factor
for rickets.
Katherine A. Dettwyler, Ph.D.
Associate Professor of Anthropology and Nutrition
Texas A&M University