A young Hispanic mother brings in her 2-month-old son. She is upset coz her neighbors has threatened to call teh Child Protective Agency coz they think his birthmark is a bruise. Her son was teh product of an uneventful pregnancy and spontaneous vaginal delivery. On examination you see a large, flat, smooth-bordered bluish mark on his buttock and lower back. Otherwise his examination is unremarkable. Wat form of birthmark is this likely to be