What causees toenails to turn black around the edges?

Many things: Many things can cause discoloration of a toe nail. It could be blood resulting from a history of trauma or a fungal infection. I would advise seeing a podiatrist or dermatologist to rule out anything more serious, such as melanoma.

Black toe nails: Black colored toenails could be from trauma, fungus , hematoma and even a rare form of cancer. See a podiatrist for a biopsy to play it safe.