A black hair on the left cheek.

It is a natural physiological change of the human body. Black hair grows because the hair follicles in the pores of the skin are activated, or because of the influence of some hormones, the skin produces extra melanin. It doesn't matter if a black hair suddenly grows on the left cheek. Don't be too nervous. If you have discomfort or symptoms, you should seek medical advice in time to avoid any risks.