It usually is dental caries, but some food stains could be there etc. We aren't supposed to diagnose without all a few tests. For one , we run a probe along the blackish area, if we feel a catch, then its caries. OPG is like teeth X-ray. You can make out carious lesions in it clearly. Caries are just blackish things on the teeth caused by bacteria due to decaying food stuff  like those that hams put in their mouth and having bad oral hygiene. In some cases, like after chemotherapy, people will have caries that they cannot help. Even kids wearing orthodontic appliances I will excuse,  cleaning them can sometimes be difficult, and some areas are very hard to reach. But fats just have no excuse.