At what stage of ringworm does a cat’s skin turn black?

Skin turning black usually means the cat is in the mid-to-late stage of ringworm. The blackening happens because Malassezia and Microsporum fungi keep damaging the cat’s skin, causing further keratinization of the dermis and the darkened appearance.

At this point, external antifungal skin ointment alone can no longer fully cure the cat; it also needs oral antifungal medication. If the lesion keeps darkening after the cat takes antifungals, the condition is not under control, and you should take the cat to a professional veterinary hospital for examination and treatment.