Mango sap causes a temporary allergic reaction that can leave dark marks. To relieve itching and inflammation, apply a mild over-the-counter hydrocortisone cream (1%) twice daily for a few days, and take an oral antihistamine like cetirizine if needed. Once the irritation subsides, you can address the dark marks, which are temporary. The most important step is applying a broad-spectrum sunscreen daily, as sun exposure darkens these spots. To help them fade faster, you can use topical creams containing niacinamide, azelaic acid, or vitamin C. Rest assured, these marks will fade over time. See a dermatologist if the reaction is severe.