Black stripe nail. This could be a mole or freckle in growth center of the nail (matrix), fungal infection of the nail & a melanoma under the nail. Needs a diagnosis. Melanoma is a dangerous skin cancer that needs to be diagnosed & treated as soon as possible to have better chances of a cure. If this is a new pigmented streak, if it is changing (darker, wider, more irregular), please see a dermatologist asap.
Usually genetic . Melanonychia striata is a possibility would need a picture also may have just damanged the nail root and could be scar changes that originate at the root level . I would get it check by a podiatrist.