Underarm darkness can have multiple causes It is not only because of lack of cleanliness that your underarms are dark ! You might be having PCOS OR PCOD . The darkness happens because body develops insulin resistance which leads to the development of Acanthosis Nigricans which is medical name for dark velvety skin that develops under the arms and round the collar ! To manage the underarm darkness the primary goal is to manage the underlying cause . There are certain dermatological interventions that can help to improve this underarm and neck darkness which is also called Acanthosis Nigricans