The phrase "black toenails" refers to a condition where the toenails become dark or black in color, usually due to injury or an underlying medical issue. Full definition
Black toenails typically develop when your toenail rubs (or jams up) against ill - fitting shoes while running for an extended period of time. (dailyburn.com)
The sexy gold high heels are open, revealing those sexy black toenails. (creativefashionglee.com)