Suggest Remedy For Itchy Rashes

hi doctor, i developed underarm rash almost 2 months ago....i was qt surprised of itching as i never get these kind of itches or rashes esp underarm becoz i dont sweat a lot and i keep it clean always (also my underarms dont stink)...anyway i thought it might be becoz of the rezor i am using..i changed my razor and applied cortizone which stopped itching and rashes, say 90 % ....but though the intensity is much lesser but still area around underarm itches few times daily..also my underarm has turned black...i am unable to find out the reason..plz help..thnx