Don't worry about your dark skin being vascular necrosis, salim. It's discoloration from the blood, and it's stained your skin.
However, if you *had* clamped too tightly for too long the tissue *would* die. It's called gangrene, and they'd have to cut the dead tissue out before it spread to other parts of your body. Believe me, if you had gangrene, you'd know it. Don't let your cock get too cold when you're clamping, and don't cut off the circulation completely. If it starts to get cold or numb, release the clamp and stimulate yourself to get the blood flowing again. Then don't do it so tightly next time.
The only thing I've heard might work for the discoloration is a dermal peel, where they use acids and shit to burn the skin off.
But here's the best advice you or anyone else could get at these forums would be to ignore any advice offered by desiel (sic) dick, or anyone else who can't spell their own name.
He just likes to pop off, so ignore him. He knows more about how to construct a sentence in English than he does about Penis Enlargement, which should tell you something. And he'll never post a picture of his withered little pee pee, either.