Welcome and WOW! at the same time
I am not a vet.
1st- I would take him to a vet asap. (since I don't know your experience- I apologize if you are very experienced) Just trying to help.

2nd- are you sure that is his tongue and not a piece of bark (substrate) he tried to eat and a thin piece went down his throat leaving the rest out of his mouth. I would make sure it is his tongue. It would have taken a few days to turn black like that even if it dried up.
Sorry if I go on and on but I like to fix problems and with chameleons, it is very (super) important to get it fixed as soon as possible, then work on the habitat and feeding issues (as mentioned by others -a lot needs to be fixed).
Reasoning:
Take to vet asap- 1 if the tongue is black it needs to be removed and treated for possible secondary infection, which may occur (I'm not sure it is the tongue). Being blunt- Gently hold your chameleon (he will probably open his mouth as many Veileds do as you mentioned "Yelling" that is normal) and touch the black object to find out what it is-- It has to be removed and I think a vet would be better-- as in Emergency Visit (I would personally just walk in since it is an emergency)
You mentioned it has not eaten in a few days-- So after problem with the black object is fixed food needs to be given (which feeding needs to be addressed with proper food items and vits/calcium.
Again, we need to get the black item identified/fixed before anything else. < I care so much it seems a little bossy, but I am just expressing the importance before something else happens. Thanks for understanding.
