This sounds like eczmea, which in dark-skinned individuals often produces temporary undercoloration. Cause is heredity, usually, not allergy. Hydrocortisone and its derivatives helps. You can try OTC 1% first. If that doesn't smooth things out, your pediatrician can prescribe something a bit stronger. The color comes back by itself, though it takes months. Ask your doctor, next time you see him or her.
Best.
Dr. Rockoff