The exact reason for your problem needs proper examination and investigations as there may be quiet a few reasons for your legs turning black. the most common it may be due to over exposure to sun if wearing shorts , there are few skin conditions also where the skin start turning dark for that u need proper evaluation. it may be assosiated with
thyroid abnormalities or
vitamin deficiencies.
Next Steps
i would recommend u consult a dermatologist so that exact reason for your problem can be pinpointed and treated accordingly. if possible kindly send a photograph so can help u better
Health Tips
avoid sun exposure use sunscreen regularly and please avoid using self medications as they will only result in worsening of your condition.