Our new pet rats...

Question
Me and my boyfriend recently purchased 2 domestic dumbo rats from a local petsmart.  We notices 2 days after having them at his house that our one white rat had a little black spot under her chin, and we are not sure if her and the other mouse got into a fight, or it is just a little genetic birthmark on her... or if we should go get her looked at.  Do you have any suggestions or anything that could help us out?

Answer
Is it flush with the skin and under the fur or is it a raised scab?

If its flush under the fur then I'd say it may just be a little pigment mark.. but if its scabby in any way yeah its probably just a little battle scar that she may be just fine with. However if it gets bigger, begins to bleed or ooze, or look otherwise red/angry/infected -- see a vet. :)