What Is The Treatment For Itchy Black Patches On The Chest?

hello doctor,i m in my final year of dentistry.....i m getting black patches in mah chest region and inguinal area and its spreading irregularly and is itching.......when i rub peels of skin is coming out......i applied diluted dettol and on drying i applied fusiwal ointment....still black marks persist and sometimes it itches a lot.....it started after i joined swimming....so kindly prescribe medicine to me and some tips to get rid of it...thank u