Hi Darkness of legs in old age is due to less peripheral circulation called as Peripheral Artery Disease. There are simple maneuver that can improve the blood flow to limbs as Balance exercise with rest, Stop smoking, control of blood sugar in diabetes (not in your case), control of weight by diet control, lowering of lipid profile with drug treatment from your physician. Most important is daily message of lower limbs with oil like coconut oil / OTC message oil which will improve the peripheral circulation. Low dose Aspirin should also be taken prophylactically to prevent any future clot formation. Take Care