Hello,
Two possibilities are coming to my mind regarding this dark, dry and itchy rash. The first possibility is of erythrasma and the second possibility is of eczema. Erythrasma is a chronic superficial infection of the folds of the skin and typical appearance is well-demarcated, brown-red macular patches. The skin has a wrinkled appearance with fine scales. Wood lamp examination and gram staining are the tests for its diagnosis. Eczema is a form of chronic dermatitis (rash).Allergic reaction is one of the important causes of eczema.
My sincere advice is to get it examined from a dermatologist so that treatment can be planned. It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.