The only way it can heal is with the healing stones.
When you reach 40,000,000 xp a text pops up saying; Whoa! You have grown so big. now you can fly-high in the Sky. (Tip: Double-tap Boost button (space bar or left-click) to take-off or land when you want!)
The Black Dragon looks like a much bigger, black version of the Dragon, but instead of the dragon wings, it has bat-like orange wings. On its back, there are three double-pointed spikes. It has a devil's tail, and it is the only animal whose eyes are shaped in ovals to look mad. Its nostrils are also in ovals, like its mythical counterpart.
Eat normal dragons to level up, keep on biting and burning it until it's dead (watch out for tail-biters when chasing the dragon). You can also troll, team up with a Fox and tell him to not eat food and stay as a Fox, then when chasing dragons the fox could help you dig them out when they go into hiding holes.
You should only stay near the lava biome, as you survive by drinking lava. Use the big Healing Stone in the lava biome if your hp drops too low. ( Also beware of the Jellyfish because its stings don't need a recharge and if a Jellyfish and other animals team up such as Pufferfish, more Jellyfish, Orcas, Blue Whales, etc their combined effort can kill you, so if you see a Jellyfish use your fire and burn it.)
As you have lots of XP, you should be wary of tail-biters. If anyone approaches your tail, turn around and attack them, and pursue them at all costs (each tail-bite makes you lose anywhere from 200-400K xp). It is best to chase them into the lava where they are weak and where you're basically the King of the Lava and you can have a nice snack...
If other Black Dragons exist, they will attack you. If this occurs, you fight to the death. (inevitable) Remember that tiers 15 and 16 can bite each other's tail, so you have a pretty good chance of making it, but beware of their fire. Try to dodge the flames and use your own fire. Don't take the risk of bargaining with them as a team unless you have 20M XP or more.
This version of the Black Dragon existed a long time ago. Very few unedited screenshots exist of it, but it might have had an ability that produced a stream of fire from its mouth. This ability is believed to have instantly destroyed any animal it touched. This incredible power may be the reason it was removed, and why the Black Dragon is hard to get in the old times.
Have you ever reached to the Black Dragon?
Current Black Dragon design
Old Black Dragon design
A Donkey killing Two Black Dragons fighting with only one kick