Generalized darkening on your belly and back can stem from various causes, including post-inflammatory hyperpigmentation, hormonal factors (like acanthosis nigricans), or a common fungal skin condition called tinea versicolor. Because the right treatment depends on the exact cause, I highly recommend seeing a dermatologist for a proper diagnosis first. If it is simple hyperpigmentation, over-the-counter lotions containing niacinamide, alpha-arbutin, or lactic acid can safely help even your skin tone. However, if it is fungal, you will need an antifungal treatment. Avoid using strong, unregulated bleaching creams, as they can irritate your skin and worsen the darkness.