I would hate to say it, but it sounds like you have to use
Deus Ex Machina. Or in other words "God on a machine." This is bringing in a hopefully believable action by something. Bring something in to distract the dragon, back write to bring in another character in to help him at this time, or something like that.
I like the bring in another fighter idea (it is use a lot), just back write somewhere to where you have befriended an archer of some type, and have them show up to fire an arrow to blind the dragon or whatever and let the main character kill it off. And just have the archer say they felt the main character would need help so he started to track him down, and just showed up at the right time.
Or you can have another dragon show up, and the dark dragon decides to fight the greater danger. Have it either die or get damaged enough for the fight to end. Then you have the problem of another dragon to deal with.
Or you can back write and give you a weapon or item to kill the thing.
It is a hacky way to take care of things but it is for the best. But hey it is a book, and it is not set in stone what is going on.