why is my tooth black and why didn't my dentist tell me

Did your dentist mention that you should get your wisdom teeth extracted sometime in the future? Without touching the darker area with an explorer, this looks like decay, however the good news, this is a 3rd molar (wisdom tooth). There are multiple factors at play here but most 3rd molars don't get much decay treatment (ie, filling, root canals) from dentists. 3rd molars are harder to keep healthy due to being further back, misalignment, partial eruption, being impacted, etc., and therefore are more subject to decay, have gum issues and could also compromise the health of neighboring teeth. A dentist would have to find these teeth to be in an ideal environment to become "save worthy". Without seeing your other 3rd molars, the tooth in question 1.) has decay and 2.) is out of alignment, therefore I'd say most dentists here would recommend extraction. A dentist may not pay much if any attention to a tooth he will treatment plan for extraction latter down the road. I'd recommend discussing this with your dentist.

Try r/dentistry?

Is this actually a serious question?