I decided to go back to the dungeon this time. I rolled a d14 (Thanks Col. Z!) to determine the party level, though I suppose I could have just rolled a d8 for the dungeon level. I got a 9, which was level 8+. So we're pretty far down in the ol' labyrinth here.
A roll of 5 on a d20 results in (drumroll)
1d4 Black Dragons (result 3)
That's right, folks. We've got THREE black dragons roaming the halls here. Granted, the DM needs to make judgement calls about whether an encounter makes sense or not, but this is an exercise so we're going with it.
It wouldn't make sense for dragons to live in the same lair. They are solitary creatures as a rule. They also don't like sharing, not to mention a single micro-environment like a dungeon would have a hard time feeding multiple dragons. But what if it was mating season?
Two male blacks have entered the dungeon seeking a female that lives in the deeper caves. The party encounters the two would-be suitors just as they have met upon each other! Reaction rolls are rolled aaand
The dragons have a 5 toward each other (Hostile, possible attack) and an 8 and a 9 toward the party (Uncertain/confused and no attack/considers offers). So the two bucks are wyrming their ways through the halls to the lower caves when they spy each other. It's obvious why each is here. Now they're circling and snarling, spoiling for a fight when the PCs stumble into the mix. Neither dragon is so stupid as to ignore a party of 9th level PCs, but the other dragon has to be its main concern. One is flummoxed by this added complication. The other is pondering if the PCs could be enticed to help him, or at least to attack his rival first. All this plus the THIRD dragon is somewhere in the dungeon, and maybe she is on her way up to see what all the Draconic trash-talk is about...
EDIT: It should also be mentioned that the two visiting dragons wouldn't have any treasure, as they are not in their lairs. Talk about a lousy risk/reward ratio!