@taz_s_824 I also don't think the tail color is related to the shed.
My gut feeling is it is related to the injections the vet gave. If the tail is working normally, the color might resolve itself or it might not. I see the tail isn't pure black, but a dark green so I think you will find it fades back to normal over time.
Reptiles have a different circulatory system (that I'm not familiar with it!) around the kidneys and the tail. I think it is called a caudal port--something port anyway. It is the reason vets do not give injections to the tail (and maybe the hind legs). I am also not sure if that structure applies to chameleons.
What injections did your vet give?
I've found chameleons can discolor for benign reasons. I've marked up animals restraining them to medicate them or applying pressure after an injection.
I honestly don't think there is anything you can do about the tail. I know when my chameleon discolored in my vet's hands (see below), he just commented on the stress coloration and wasn't concerned. It was dramatic and happened in his hands during an examination. (He's board certified in zoo medicine.)
This was a wild caught that had the spinous processes of his sail fin on his tail degloved and stripped to the bone at import. He's been battling infections in the tail in the more than a year since import and has been into the vet many times over it. At one point, his tail went from normal green to half green and half black while the vet was examining him. The division between the black and green was along the spine, so the right side of his tail was jet black and the left green. Over a few days or weeks, the green part of his tail went black and he ended up with a line between the black and green similar to your boy's. My chameleon lost strength in his tail which ended up almost limp. I expected it was the end of the line for him but over months, the tail slowly lost its jet black color and he is slowly gaining strength in it. I'm not sure if the infection is cleared, but the vet is really quite happy with it and the swelling is slowly going down. His tail is stronger but still weak.
Hope that helps.