The skin on your lower legs may be getting darker due to friction, poor blood flow (venous insufficiency), dryness, or skin conditions such as eczema, pigmentation disorders, or a fungal infection. Since you have a heart condition, do not use any medicines on your own and consult your doctor to identify the cause before starting treatment. Until then, keep the area clean and well moisturised, avoid tight clothing or repeated friction, protect your skin from the sun, and monitor for pain, swelling, or rapid changes.