Black pigmentation from toe till knee. What could be the cause? No itching/ burning sensation

Skin color changes. You would want to look at the black pigmentation and ask yourself. How long has it been there, has it grown over the past week/months, has it changed in size/color, etc. There are many different reasons to have pigmentation on your skin. You want to be weary of any new or changing skin lesions, especially if you have a hx of skin cancer. See your pcp/dermatologist for follow-up care.
Stasis dermatitis . Could be from iron deposited in skin from damaged red blood cells related to varicose veins. Your age does not correlate with this however. You should see a dermatologist. There are other rarer disorders that can cause.

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