Any kind of discoloration that pops up on a toenail, even if you think it's caused by your nail polish (or its often too-harsh remover), can be alarming. Healthline notes, however, that a black toenail specifically may signal an underlying medical condition such as anemia, diabetes, or even heart or kidney disease. 

Likewise, fungal infections can sometimes cause your toenail to turn black due to debris build-up. Toenails are particularly susceptible to such infections because they thrive in moist and warm environments, such as those created by your sock-and shoe-covered feet.

Most worryingly, melanoma, the most serious type of skin cancer, typically presents as a dark brown misshapen spot, which may arise under a nail bed. 

A dark spot on the toenail isn't always the worst-case scenario, though. If you've hit your toe recently, the trauma to the toenail can cause blood vessels underneath to break, resulting in bleeding — this can look black underneath a nail.