What causes black lines in the toenails?

Sometimes this: Is a normal variant, sometimes it could be blood that is dried, melanoma at times needs to be ruled out.

Various things: In people with skin of color it may be a normal variation. Skin growths may also cause them, both of the benign and malignant kind. See your dermatologist.

Different causes: Black stripes down 1 nail may be subungual hematoma. Red or brown streaks may be splinter hemorrhages. Brown/black lines from nail plate to tip are often seen in people with dark skin & are normal. Subungual Melanoma can cause nail ridges & dark colors of cuticle. It most often effects the thumbs or big toes. Dark lines may be due to other illnesses. Get checked out when in doubt.