Black spots can occur to your skin by many reasons. Reasons like mosquito bites, or any other insect bites, after some time become visible. And your leg is much more prone to such bites compared to the other body parts. There are many solutions for such things as these marks are not at all permanent in nature. There are many types of ointments and creams available in the market today which are proven by many. Other than that also maintain personal hygiene by making sure you wash these marked places on your leg on a daily basis. For more you can consult a skin specialist for the same.