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Black tongue can be a side effect of some meds like Pepto Bismol, but with your soreness and dry mouth I think it is more likely that you have developed an overgrowth of yeast or bacteria in your mouth.
http://www.mayoclinic.com/health/black-hairy-tongue/HQ00325 Here is a link from the Mayo Clinic that you may find helpful.
I would also consult a physician due to the discomfort that you are having with it.
I hope this helps. Let me know if you need more information. I will be happy to help. T