The darkening of the underarms may be a physiological phenomenon, or it may be a disease recovery caused by hyperpigmentation, acanthosis nigricans and so on. 1. Physiological phenomenon: for example, skin cleaning is not in place, or the local area is often exposed to the wind and sun, which will cause the skin to become black. 2. Hyperpigmentation caused by disease recovery: for example, after the recovery of dermatophytosis, eczema and other diseases, it may lead to hyperpigmentation, causing the darkening of underarm skin. 3. Acanthosis nigricans: related to endocrine and obesity, it may also cause local mucous membrane hyperkeratosis and hyperpigmentation, thus leading to black armpit. There may be other reasons for the darkening of the armpits, you need to go to the hospital for examination first, to clarify the cause of the disease and follow the doctor’s instructions for treatment.