Hi.. Thanks for the query.. As per your complain itchy rash along with eruption of cluster of bumps over the skin that subsided but there has been sore skin that turned black can be most probably due to CONTACT DERMATITIS that is an allergic reaction which occurs when skin comes in contact with certain agents that your body is allergic to like skin cosmetics, detergents, soaps, fragrances,leather, latex, poison ivy etc. -Another cause can be food allergy or side effect of medicines.. You should consult an Allergist and get evaluated and a thorough clinical evaluation and investigations like blood tests to rule out allergies like patch test, prick test, RAST test etc can be helpful. As the symptoms have now subsided it is fine but if there is recurrence of itching and bumps you can take anti allergics like benadryl for relieving itching and do cool compresses over bumps.. Apply calamine lotion to soothe the skin.. Once the cause is ruled out it will be treated accordingly.. In case of Allergic reaction you will be advised anti allergics, topical steroid ointment application and also avoiding exposure to allergen when it is ruled out... In recurring conditions Immunotherapy will be advised to you.. Hope this helps.. Regards.