Black toenails can be caused by excessive formation of melanin pigments. Toenail discoloration is found in some circumstances, such as pinta diseases, vitamin B12 deficiency, cancer, and Peutz-Jegher syndrome. Besides above circumstances, toenail fungus can also cause discoloration of the nail.
Fungal infection Candida albicans and Blastomyces dermatitis often causes black toenails. Junction Nevi can also be the cause of this nail disorder. Toe nail color changes in Cushing’s syndrome are also common. This is caused by an increase in the hormone MSH (Melanin Stimulating Hormone).
Some researchers also reported that nail infections by Proteus mirabilis bacteria also cause a black color on the nails. These bacteria produce hydrogen sulfide. These compounds will react with metals in toenails and produces a black color.