My toe nail has been bruised for nearly 3 months. What else could cause a bruised appearance?

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.
Toe nail. Toe nail bruising can be seen quite often in long distance runners who impact the toe against the tip of the running shoe. Often seen in marathon runners. A blackish singular line in a toe nail should be evaluated by a dermatologist for melanoma.