My son had dark circles under his eyes when he was younger.  It was during a time when his CAH wasn’t managed as well as it could have been.  We had to change doctors several times because of this issue.  I do remember one time when I forgot to give him his morning meds and sent him off to school.  By the time he came home he had horrible dark circles which he kept for several days.  I think this is the body’s way of telling us something isn’t right whether it be CAH or if they are coming down with something.