What Could Dark Skin Spot Son Spine With Bruises Suggest?

Hi, may I answer your health queries right now ? Please type your query here...my son has a darkened skin spot that is about 3 inches long on his lower spine. it looks like a bruising from rubbing on his spinal bones, however, when we recently removed gluten, dairy, and sugar from his diet...the 'bruise' disappeared after a month on the diet. For Christmas, we stopped the diet and I noticed that the bruising is back.