What Causes the Whole Tooth to Turn Black Under a Temporary Crown?

If the tooth turns black under your temporary crown, it is because of micro leakage, and the fit of the temporary crown is not perfect. Most often if you used peridex or chlorohexidine, also or stained foods and drinks, that also contributes to color of black under temporary crown.

Actually this is not uncommon.  Somehow your temporary crown loosened up a bit, probably from chewing on it, and this allowed a microscopic space for bacteria and fungi to set up residence under the temporary crown.

When the dentist took of the temporary crown there might have been an annoying odor and the tooth would have appeared black.  It is often times known as "black scuz" to dentists (not a technical name).

The black scuz and odor can easily be removed in about three seconds by bathing the tooth with regular old hydrogen peroxide solution.  No harm, no foul.  No permanent damage either.

You should return to the dentist to have the temporary checked as it is not sealed well.  Leaving the temp with the black stain could lead to a lot of sensitivity on that tooth once the final restoration is seated.  The black stain can be removed from the tooth with pumice and peroxide by the dentist