A new dark stripe, changed color, or a detaching nail may require medical evaluation. The cause cannot be determined from a photo.
Where to start?
Do not cover unclear changes with a new coating before a medical evaluation.
When to see a specialist?
A new or changing dark stripe and a detached nail should be examined by a dermatologist.
More: American Academy of Dermatology
This provides general care information. Skin, hair, and nail diseases are diagnosed and treated by a doctor; a salon procedure does not replace treatment.