Its just like Vodevil said. Its not likely that it is tail rot. My beardie's tail is like that too, and I know for a fact she doesn't have tail rot. You should keep an eye on it anyways to be sure.
If you haven't taken your beardie to the vet yet, however, you should anyways to check up on his health. Give him a week or two to settle before taking him in. You should also take in a stool sample when you do so that you can test for parasites and the like. Like I said, though, you should wait so that stress doesn't cause the parasite counter to seem to high when its likely just fine.