OK people,a clinical talk....I love talking shop....any way here are my explainations for both situations:
1)Natural teeth:
-on losing vitality, mainly due to trauma, could be because of decay
-excessive use of mouthwash,specially hydrogen peroxide.Not really black but almost there
-stains,tartar etc
2)PFM crowns:
first thing.....they don't turn black,they start appearing black. this is becausing of thinning of the ceramic layer.This mainly happens when the tooth is under-reduced.Then the thickness of the ceramic has to be compromised upen,as that of the metal has to be sufficient to withstand strength.
sometimes they appear black at the gingical margin.MAinly happens if margin is not sub-gingivally placed in endo treated teeth.
Corrosion of the metal and amalgam is also a possible reason.
If amalgam is used for post-rc filling in place of resin/GIC and a crown not made,the tooth appears black due to leakage of mercury into the dentinal tubules.
does this solve your doubts?