Usually when both the beard and the tail darken, it's not a problem like "tail rot" at all, as when they
black beard, for whatever reason they are black-bearding, their tails also turn black. So no, this is probably not at all "tail rot".
Has he had an injury or retained shed on his tail? In order for them to develop "tail rot", they have to have had either an open wound to the tail that got infected, or they have to have something cutting the circulation of to their tails, like a band of retained shed around the tail, which causes the portion of their tail below the circulation barrier to become necrotic and die. "Tail Rot" is simply a general term for an infection in the tail, usually bacterial in nature, that causes the tissue below the infection to become "necrotic" (tissue turns black and dies), and unless they get medical intervention with antibiotics the infection continues to spread up the tail towards their body, killing all of the tissue along the way, and eventually it will reach their bloodstream, they become septic, and will eventually die without medical intervention. That being said, "Tail Rot" is not something that just starts on it's own out of nowhere. He'd have to have had an open injury or circulation barrier of some kind first, for a while, and you'd be aware of that.
Again, you can post a photo for confirmation, but if he was also black-bearding then it's just a result of the
black beard. Now as to why he's black-bearding I don't know, that is caused either by pain or discomfort of some sort, high stress caused by something, or hormones.