yeah...um, i don't think it was the Atkins diet that turned your tongue black--that's crazy! people have been doing this diet for much longer. And hello, before vitamins were discovered in the 1920's, no one took supplements/ate fortified foods....i'm sure its not a vitamin deficiency

that said, there could be lots of other explanations for why your tongue turned black--i would see another doctor! even some medicines/foods can have this effect ie. did u know taking the pepto bismol tablets turns your tongue black? there are also some conditions-- i once went to the doctors the day after i took those pepto bismol, and forgot to mention it to her...she started explaining some diseases to me until i broke it and told her what had happened best of luck! =) and congrats on the weight loss thus far!