What causes skin darkening under the arms for females?

Underarm darkness. There are a range of causes for underarm hyperpigmentation, including frequent wear of tight-fitting clothing around the arms that causes constant friction in the armpit area; adverse reaction to certain deodorants; shaving; excessive weight; and dark underarm hair.
Sweat, friction. Color is caused by combination of sweat, friction, and inflammation. Antiperspirant may help as well weight control. Peeling creams such as lac hydrin or urea bases creams can slowly lighten the dark sin in this area.