Hello,My name is XXXXX XXXXX I'm a biologist with a special interest in reptiles. I'm sorry to hear of this incident. There are a couple of possibilities for what this might be. This usually means the tail has gotten infected. A condition called dry gangrene is common. Sometimes it's the result of a minor injury, but often no original cause can be found. Because the infection can keep spreading up the tail to the spine (which is life-threatening), it's best to have it treated by a vet. This link will take you to a directory of reptile vets:http://www.anapsid.org/vets/index.html#vetlistYou can read more about tail infections (scroll down) on this page:http://www.anapsid.org/gangrene.htmlIf you happen to use a heated rock in the cage, this may be an actual burn. Even if a heated rock is maintaining the temperature it is set on, burns can result. Sometimes a lizard can go for years without a problem, but often it's only a matter of time before a burn occurs. A burn would cause the skin to blacken on the part of the body that was on the rock too long. That could include the upper side of the tail if the iguana leaned against the side of the rock. Pet stores recommend these rocks, and at first glance they seem like a good idea. Pet store staff get their information from literature put out by the manufacturers of heated rocks, and they care about profit, not the animals' welfare. Here is a nonprofit site where you can read the truth:http://www.anapsid.org/hotrock.htmlRegardless of whether this is infection or a burn, veterinary care will be needed. There is no effective home remedy for either condition. If you have more questions, let me know in a REPLY. I hope your iguana will heal uneventfully.AnnaMy goal is to provide you with excellent service – if you feel you have gotten anything less, please reply back, I am happy to address follow-up questions. Please remember to rate my service only after you have all the information you need. Thank you!