The black spot could be blood from dropping someting on your toe or it could be fungus that grows under toenail. See a doctor if it does not clear up soon.
Usually, a black spot under the toenail is indicative of an injury to the nail bed. This can be either a recent injury that has bruised the nail bed, or an old injury to the part of the nail bed that lies unseen under the skin and has grown out.
It could be several things. You could have a bacterial infection, fungal infection, skin disorder or an injury. If an external object causes enough pressure to break the surface capillaries, a blood clot can form under the nail and should fade as the days pass. If it's fungal, the nail may be thick, brittle and will require prescription antifungals.