Blues are known for their Machiavellian minds. Even such a young dragon should be about ready to start on his first real foray into bending the world to his will, and the pcs just gave him a pet project to work it on.
He is inexperienced, so have him start off small. Follow them to town, observe where they frequent, and who they meet. Once they leave town again, have him start working on subverting the pcs friends and allies, taking over their places and making them his own. When they return, it will be to a much frostier welcome. Over time, he will learn subtlety and finesse, but for now it will be direct strikes against them.
If he is discovered, he should flee straight away, abandoning his schemes...his defeat was too recent for him to have regained his confidence...but he is rebuilding. When he has the chance, he will send 3rd parties against the pcs, but direct confrontation is to be avoided.
If the party moves to another town or city, the dragon will wait about 6 months before realising they aren't coming back and will track them down and start over bedevilling them.
Let them catch an occasional shadowy glimpse, even have it send them an occasional missive, particularly after a success, of the dragon gloating. "Bwa ha ha ha, suffer my vengeance, fools." In scratchy crude lettering on parchment.
Not enough hassle to make the players hunt it down and kill it, just enough to annoy them occasionally.