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.
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. ;)
Could be a Port Wine Stain(PWS) birthmark. Take a look at the Vascular Birthmark Foundation website: birthmark.org
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.
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.