Why is my toenail darkening on the sides? Only one toe has a nail that's turning dark in the corner. Is it a sign of a fungus or caused by my shoes?

Dark toenails. Dark lines in toenails of African Americans is often a result of genetics, but can also be triggered by chronic minor trauma, such as shoes as you have mentioned. Fungal infections of the toenails is usually characterized by yellowing, thickening and flaking.