I did see that mentioned as a theory a few weeks ago. My best guess is that in the United States especially, it's a combination economic/socitial factors and vitamin D deficiency that's making the virus have higher mortality in populations of racial minorities with darker skin.

Here's a study that looked at vitamin D levels and found that they're significantly lower in both people with darker skin, and (independently of skin color) those who are overweight. https://www.jabfm.org/content/29/2/226