Hi, The dark pigmentation near your buttock area is known as Post Inflammatory Hyperpigmentation/ Post Inflammatory Hypermelanosis (PIH) due to the activity of melanocytes which are the color producing cells. It is a condition of the skin arising from over activity to the skin resulting in healing with pigmentation. If I were your dermatologist then for dark pigmentation I would have suggested you to use azelaic acid cream or Kojic acid cream or glycolic acid cream or licorice extract or arbutine creams at night time in guidance of dermatologist. Most importantly you should reduce your weight so that there should be least of friction causing the pigmentation. Take care.