I'm sorry I haven't had the chance to respond to you all until now. Thank you all so much for your helpful replies. I was told it is my Mum's medications, that caused her teeth to go black. Her nursing home don't have a visiting dentist, so I asked a local one for advice. He said, my Mum's teeth are obviously very decayed & if she got an abcess, then she would need treatment.
I hope that won't happen, as it would be very traumatic to take her to the denist. Thank you all again.