The dark skin around a piercing is a normal part of the healing process and will fade with time. However, some piercings can develop a scarring or keloid in the area around the piercing that turns it darker. This can happen when you wear metal jewelry that reacts with your skin, or if your piercing gets infected. The best way to lighten this type of scar is to take good care of the piercing, and to wear only stainless steel or gold jewelry that won't react with your skin.
A keloid will look like a raised bump with darker skin than the rest of your body, and it can be painful, itchy, and uncomfortable. If you have a keloid in the area of your belly button piercing, you should see a dermatologist to learn more about treatment options.
There are several ways to treat a keloid, including surgical removal and radiation treatments. Some people also use topical corticosteroid creams, which are available OTC. These help reduce the inflammation associated with keloids. Other treatments include using a silicone gel that's designed to reduce the appearance of stretch marks and scars.
Another natural way to lighten the skin around your piercing is by applying a chamomile soak. Bang Bang Body Arts, a custom body piercing studio in Massachusetts, recommends chamomile in their aftercare guidance. The chamomile helps speed wound healing, and research has shown that it can reduce the appearance of scars. Mix 1/4 teaspoon of iodine-free salt to 8 ounces of warm water, and apply the solution to the piercing site twice a day for 5 to 10 minutes.