Blackish color gum in my daughter's mouth!!!??? What is it?

I brush my children's teeth just to make sure they are clean. During the mid day they will brush their own to get practice. As I was brushing my daughter's teeth today I noticed it looked darker on her gums. I am actually concern about it, I don't know if I means something or could be normal. Obviously I'm not an expert when it comes to this. Has anyone seen this or has expiernce it personally or your own child, a friend, or loved one?