I have noticed a black mark (best as I can describe it's like a freckle on the gum line) on my 15 month old's gum at the front, it's at the top of the gum by a tooth that came through last August so I don't think it's anything linked to tooth eruption.
It looks like it is on the gum, as opposed to on the tooth underneath or is food stuck or something) and unfortunately make the mistake of googling it and am now convinced DD has some sort of genetic disorder.
I'll be calling the dentist in the morning (v lucky to have a private dentist that sees children as NHS patients) but just wondered if anyone else had come across this. When I Google, aside from rare genetic disorders it is either really vague or talks about marks on teeth, and I genuinely don't think it is decay.