Hello,
These black lines can be due to can be due to longitudinal melanonychia(presence of a pigmented stripe, usually brown or black, along the length of the nail bed in darker-skinned individuals,splinter hemorrhages or Beau’s
lines(they are horizontal
lines of darkened cells and depression).Without examination, it is difficult confirming a diagnosis and so I would suggest a dermatologist’s evaluation.
I hope it helps. Take care and regards.