It typically happens due to trauma or injury to the toe, such as stubbing it or dropping something heavy on it. The blood trapped beneath the nail causes it to turn black or dark in color as it dries ...
And then, a man with a shaved bald head and sunburned scalp told me that if I was lucky, and if I trained hard enough, my toenails would turn black. He revelled in the pride of his own bruised nail ...