Dentist Appointment

I had to have a root canal done so today I got my crown put on. Which is no big deal, but while there my dentist showed me my latest x-ray. I have a black spot under that tooth(the root canal one). Ok so he says this and all I think about is BAD BAD, I tend to do that huh!??! Anyways he explained he had never seen this before and at first was a little worried and actually was getting ready to call me to have me come back in so we could remove the tooth. But first he went to his dad, who is also a dentist, and found out he had patient that had the same thing. So they compared x-rays and they look identical. I already have an appt. with an oral surgeon to have some other stuff done, so they're going to check this out too. More than likely he said, it's fine that it's probably just a void. I really hope it's just a 'void'.