What are the causes of a swelling foot and leg and turning black?

Mutliple causes: Your symptoms sound like there is something going on in the blood vessels where tissues are not getting enough perfusion. Multiple sources can cause this including hx of smoking, diabetes, vascular diseases. But turning black is not good sign and warrants urgent attention. Have your doctor take a look and made need further specialty care.

Vein disease common: Venous insufficiency is a common cause of swelling and hyperpigmentation (blackening). This circulation problem is often easily curable although the cosmetic damage may not be completely reversible. A simple, painless and relatively inexpensive ultrasound test can determine if this is the cause. A phlebologist or other physician experienced in this area can quickly assess this problem.