Cause of toes turning blacked?

Nails or toes?: If you mean the toes themselves, this is a little worrisome. Usually the skin will turn black if there is tissue death--necrosis--of the skin. This can happen if blood vessels are damaged or inflamed. Sometimes, blood under the skin can cause this appearance as well. If you mean the nails, that can be a sign that a mold has gotten under the nails. Either way, i recommend a doctor visit.

Lack circulation: You may have vascular problems causing purple or black toes. Consult vascular surgeon immediately or go to the er.