My dad's tooth is turning black...it has previously had tooth filling...what does it mean? what should he do?

Black tooth: If the tooth has on old amalgam ( silver) filling, the silver can leach into the tooth when it oxidizes and this will turn the tooth black. He may also have decay. He should see a dentist and decide what needs to be done.

Black tooth: Black tooth usually means that the nerve to the tooth has died and is no longer viable.

Dental evaluation: The tooth discoloration that you describe could be from a few things. An existing amalgam filling over time can stain the tooth and give it that "blackish" discoloration. It could also be from caries (cavity) or the nerve of the tooth might have died causing necrosis of the internal tissues of the pulp chamber. It's best to see a dentist to determine which one it is and what treatment is indicated.