Other way around. Darker skin in North America is correlated to more cancer, due to the consequent vitamin D deficiency.
Most of the population is now vitamin D deficient because people don't spend much time outside.
Most of the population is now vitamin D deficient because people don't spend much time outside.
It may not be simple quantity of exposure, but rather severe sunburns that most elevate risk. If one is outside regularly and acclimatized, these severe burns are less likely to occur.
Perhaps someone else will have a more detailed comment on this.
Of course, one's pigmentation will also influence the likelihood of sunburn, although dark pigmentation does not entirely prevent burning.
EDIT: Just read InclinedPlane's comment, above, which mentions the same concern with sunburn.