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Blackish discolouration can be caused by many causes. Few of the causes is pigment disorders, also fungal infection
. You need to keep the area clean and dry. Avoid sweat accumulation. Avoid sun exposure. Eat plenty of fresh fruits and green leafy vegetables. Drink plenty of water. Consult a Dermatologist
to find out if it is infection or other cause. The pigmentation can be treated by good skin lightening creams. Do not apply any cream now as it could make it worse later. Please consult your Dermatologist who will be in best position to help you with good treatment options.