Nothing is more embarrassing than seeing somebody else's underarm darker than it's actual complexion. It looks so unkempt and messy meanwhile these people might not know the cause. Darkening is commonly found in the armpits, back of the neck, groin, elbows and knees. Darkened armpit is sometimes refer to as acanthosis nigricans medically. Before we go into the home remedy for dark underarm/inner thigh let's look at the causes.