This article talks about it and other possibilities known to be investigated 10 days ago - the list could be incomplete by now. https://www.sciencemag.org/news/2020/04/how-does-coronavirus-kill-clinicians-trace-ferocious-rampage-through-body-brain-toes
It's caused by inflammation of the blood vessels, or vasculitis, or maybe blood clots - they aren't really sure.
This has some good images of what it looks like on different skin tones -
Mom, just read an article this morning in Washington Post that talks about the toe thing. It can also be other parts of the body. It's like frostbite. Apparently Lupus sufferers get this, as well as other virus sufferers. It's a rash. Dermatologists were interviewed for the article, and all info here comes from them. They think it's part of the immune system response. It goes away in a relatively short amount of time. It happens almost exclusively in younger patients who aren't severely ill and all have recovered, at least according to what I read.