I think it's tooth decay. My 4 yr-old niece & a student's teeth had blackish edges. Both kids' front teeth were "eaten up" by decay. My niece had veneers (fake teeth) attached so that she had a proper smile. My student had most of her front teeth taken out via GA cos less than a quarter of each tooth was left by the time she hit 5 or 6 yr-old. She had autism so it was hard to do dental work on her.
Think you better find a dentist good w babies & kids.
My bro recommends Tooth Angel for my 5 yr-old nephew. My niece went there too. The dentist's chair has a TV screen for kids.