Or just go ahead and change the power: make it an effect which heavily obscures the area instead of totally obscuring. The characters will still need to be adjacent if they don't want to take the total concealment penalty, but if they are next to the dragon then its only -2 for concealment instead. Much simpler, much more balanced and it should make the fight difficult but still fun, whereas right now the Black Dragon is, as you've pointed out, much too difficult to kill.