I can't believe that someone else is describing exactly what happened to me when I was an adolescent. At about age 10, I started getting dark spots on my legs and ankles, and later they were all over my arms too. Talk about humiliation in gym class when we all had to wear T-shirts and shorts! I was taken to many dermatologists who ran many tests, prescribed numerous creams and ointments, etc. and nothing ever helped. They didn't itch or anything; they were just ugly and unsettling. When they first appeared they would almost look like hickeys, like blood vessels were bursting under my skin or something. Then, when I was about 15, they started to go away for the most part. But today I am 27 and even still have a few remaining spots around my ankles or on my arms. To this day I have no idea what causes them.
But I did recently find out that I have endometriosis, and I also have a number of other autoimmune problems like arthritis, allergies, etc. so it wouldn't surprise me if the spots were an autoimmune problem too. But there's really no way to tell; all doctors have ever done for me is look at them, scratch their heads, and tell me to live with it.
Please let me know if you get any new information about this problem, or if you'd like to talk more about it. I just still can't believe that I'm not the only one this has happened to...