When a person is said to have a black tongue, it is a metaphor comparing them to the Tibetan king Langdarma who was said to be exceptionally evil and cruel. They may also say you have the black tongue of Langdarma himself. Some people have conditions where their tongue has black spots or excessive charcoal use can cause the blackening of a person’s tongue, but most people view this saying metaphorically or spiritually as opposed to considering an actual black tongue as a sign of evil, because there are not enough black tongues in the world for that to be accurate.
To see black tongues with your third eye, simply focus on a person and envision them in your mind’s eye. Try practicing on someone you know would have a black tongue like R. Kelly or Bill Cosby. You may also notice that your third eye gives you black tongue visions depending on your own morality. Some things may be illegal in modern society for some reason, but the black tongue vision is not based on the legal systems of some random country that you were born into or took a flight into. It is based on your ideas of what actions you would consider evil. This is also relative, and if you are infallibly good or a serial killer then your sense of morality will be too distorted and impractical to actually get much use out of this skill. You can also tweak this vision depending on the action, by ignoring certain actions you know or suspect a person of having done. But when you are choosing to ignore something and still see a black tongue on someone, that means they do multiple things you consider immoral.
The black tongue vision doesn’t mean you should disconnect or confront anyone but just be wary of those you get visions of with a black tongue in your mind’s eye. This will not work in scripted shows because you will only see the actor, and just because someone has a black tongue you can see does not mean you will ever realize or care what the actions they committed are. Even though it may seem fluttering or discomforting since you only see the tongues and never explanations, but you are still better off than you would be without the knowledge.