I understand your concern but 1. The information provided about the patches is insufficient to comment on . Black patches can be due to a lot of reasons and only upon examination can a dermatologist diagnose and guide you . 2. Looking at her age, it is better and safer for you to take her to an allopathic dermatologist near you. Because if she needs medicines which need to be applied , then she ideally should be examined first.
Visit an allopathic dermatologist near you .
Do not try home remedies to avoid irritation to skin.