So I was telling my sister what happened when I woke up with a black tongue the other day (see this post). She told me to go on Google and search it. So I did because I hadn’t done that yet. I came across an article on Wikipedia that said:

Pepto-Bismol has been known to cause one’s tongue and stool to turn black. This form of “black tongue”, however, is a result of the separation of bismuth from salicylate, forming bismuth trisulfide (Bi2S3) in combination with the sulfur found in saliva.

Bingo! I took a Pepto-Bismol chewable tablet before I went to bed which resulted in me waking up with a black tongue in the morning. Woot!! Now that mystery is solved. LOL!!

[Edit:] Brushing your tongue with a toothbrush and water helps remove the blackness.