I recently started to eat just sunflower greens by itself for breakfast sometimes and noticed that my tongue gets very black after and it lasts for a few days. My back teeth also get a little black. At first I thought maybe it was just dark green from the sunflower greens, but on careful inspection it always looked pitch black to me. And anyway, sunflower greens aren't even a real dark green and definitely not as dark as my tongue gets. I researched this on the internet and found that a black tongue can be caused from a bacterial/ fungus overgrowth or a chemical called bismuth reacting with sulfur in my saliva or gastrointestinal tract. If it were a bacterial or fungal overgrowth I would think that my tongue would always be black, but I haven't noticed it happen with other foods. Bismuth is in pepto bismol and so I bought some to see if it would cause my tongue to get black from any possible sulfur in my saliva or tongue. I put some pepto bismul in my mouth and swished it around and held it a few minutes and then spit it out. My tongue did not turn black. So I guess when I get a black tongue from sunflower greens it can't be because of sulfur in my saliva or tongue. Next time I might try swallowing the pepto bismol to see if that would make a difference. I called the company that sells the sunflower greens and they never heard of anyone getting a black tongue. They told me that maybe I am having an immune system reaction. I don't know, it seems like a plausible theory because later I also found information on the internet that black tongue may be associated with a compromised immune system. Does anyone know anything about this?