when the skin in the under arm gets irritated or get infection dark pigmentation occurs. Usually if one shaves their under arm and before twenty four hours uses soap or deodrants there chance for darkening. Other reasons can be too much of moisture causing rash in under arm which has to be treated immediately , even climate is a reason
Due to shaving it could be subsurface hair, buildup of dead skin cells, antipersperant and deoderant use, A medical condition called acanthosis nigricans, or hyper pigmentation.
