bromine is a famous element that was used as poisonus gas also arsenic was used it is a more famous one that was more commonly used
Bromine
Arsenic is the famous poison that turns your tongue black.
your tongue turns black
Black light
Tannin is a natural chemical found in acorns which is poisonous to humans. When you boil acorns in water, the tannins are released and turn the water black. this could possibly turn you tongue black if you drink it.
These are silver halides as AgCl and AgBr.
the heating element turns black because it heats up all the water xxxx I think this is likely to be because your water supply is acidic (pH less than 7). This can dissolve the element over time. Probably best to replace the kettle with one with a concealed element.
It means there's no blood transmission to the tongue.
It turns into another element.
Copper (Cu) is the element that turns green when exposed to moist air.
Chow chows tongues are blue/purple. You should search some pictures of them.
This is a good description of what the tongue does.