Dark underarms may simply have to do with pigmentation and levels of melanin in the skin. But sometimes armpit discoloration ...

Hyperpigmentation is the abnormal darkening ... causing an itchy or burning rash that is moist and glistening with a scaly border. Obesity and diabetes further add to the risk.

Hyperpigmentation is rarely harmful ... if you have pigmented spots that develop in areas that didn't have a rash or previous marking. That could be cutaneous T-cell lymphoma, says Dr.