However, it’s uncommon for them to cause an itch without a visible rash or skin changes. Itchy skin with no rash can be confined to a small area of your body, such as a hand or your scalp ...

It appears in the skin folds of the armpits, groin, and around the neck. It's not common, but an acanthosis nigricans rash can be itchy. Acanthosis nigricans is linked to increased insulin levels ...

Erythrasma is common in the armpits, under the breasts, in groin folds, and between the toes. Depending on the skin tone, the rash is reddish or darker colored and looks coral/red under a black light.