Dont worry it sounds like a birthmark. To be on the safe side though the next time she has an appoinment ask her peds about it and ask if you could get it tested. If your really worried about it.
My daughter had a red mark on her head after birth for a couple months because she liked to rest her head against the bone. It could be that. Best thing would be to ask her doctor.
Could be a Port Wine Stain(PWS) birthmark. Take a look at the Vascular Birthmark Foundation website: birthmark.org
I would guess a birth mark if it's been there since birth....TMI, but I have a small round purplish/bluish/bruise looking like mark on my backside. Been there forever. ;)
Maybe her birth mark. Or does it look like a bruise? If so it might be a Mongolian spot. Google it, it is more common with darker skinned babies.