Sorry, I just saw this question...when it's healed it should look like the end of his tail is simply closed up and doesn't have anymore dry, flaky skin or scabbing, and no more black tissue. That's the key, as the black tissue is the necrotic tissue, and that's what you have to watch carefully to be sure it's stopped progressing up the tail towards the body.
The problem is that their tails have very poor circulation, so when any part of the tail becomes opened up it easily becomes infected, because there isn't much blood flow to deliver the blood cells to fight off the infection. And that's also why they typically just do an amputation of the tail if it becomes infected and the tissue is dying, because even giving them antibiotics doesn't usually help because there isn't enough blood flow to deliver the antibiotics in the blood stream. So they just do an amputation about a 1/4" above the necrotic tissue, put a stitch or two in the end, and medication directly with antibiotics. That's typically the only way to stop the "Tail Rot"...