Black toe nail
The only serious problem you could have, that gives a black toe nail, would be a malignant melanoma
on the nail bed beneath the nail.
A bruised nail turns red, and then dark, and then resolves after a period of time, as the nail grows out. It may take more than 3 months for a bruised big toe nail to grow out, so it may pay to be patient and wait, or visit a dermatologist right away for help.