Hello, I can understand your concern for dark marks on the arms after the bump formation. In case of any inflammation due to bumps there is over activity of the color producing cells known as melanocytes. These produces higher color on exposure to sunlight too and therefore can cause more darkness of skin. In most of the patients it is self resolving condition and gets better by itself. In my patients I suggest them to apply a sunscreen solution for few weeks of period. In most of the patients it is sufficient and the skin gets peeled off to get normal color. For topical application if I were your treating doctor then would have prescribed you a gel containing Kojic acid cream to be applied at night time. For peeling you need to get proper guidance from the treating dermatologist. Do not apply any other medicated cream without medical supervision. Avoid situations of direct continuous sun exposure for long hours. All the best.