I never heard of Pepto-Bismol turning one's tongue black, but I haven't taken it in years. Anyway, I Googled, and it turns out, it's not unusual!

This is from the Pepto-Bismol FAQs:

I noticed that Pepto-Bismol sometimes darkens the tongue/stool. Why does this happen and how long does it last?
The active ingredient in Pepto-Bismol contains bismuth. When a small amount of bismuth combines with trace amounts of sulfur in your saliva and gastrointestinal tract, a black-colored substance (bismuth sulfide) is formed. This discoloration is temporary and harmless. It can last several days after you stop taking Pepto-Bismol. Individual bowel habits, your age (the intestinal tract slows down with age), and the amount of the product taken all help to determine how long Pepto-Bismol is in your system.