To unlock the Blimp? Blimp. trophy in Cities: Skylines, you will need to purchase and install the Mass Transit DLC.
Blimps become available once you reach the Big Town milestone at a population of 1,800 - 7,500 depending on which map you start with.
Once you have blimps available, buy a blimp depot (20,000) and 2 blimp stops (16,000). Create a blimp pathway between the depot and the two stops by selecting each one and looping back around to the beginning to complete the line.
Repeat two more times and the trophy will unlock. Repeat a further 7 times and you'll also unlock the trophy below: