What Causes Large Birth Mark On Shoulder Blade During Puberty?

My daughter is 26 and during puberty developed a large birthmark, darker in color on her shoulder blade and down her arm, stopping just above her elbow. The hair is more coarse on her back where the birthmark is. A dermatologist diagnosed it 15 years ago, he stated it is a condition Japanese girls can get during puberty. The diagnosis had a Japanese name, I cannot remember it, are you familiar?