John Blake's (Joseph Gordon-Levitt) legal name is revealed to be Robin at the conclusion of The Dark Knight Rises — but that doesn't mean he is the DC Comics version of Robin or that Blake becomes the costumed Boy Wonder or Nightwing after the film is over. Released in 2012, The Dark Knight Rises is director Christopher Nolan's epic conclusion to The Dark Knight Trilogy in which Batman (Christian Bale) comes out of his self-imposed 8-year retirement to save Gotham from a new threat, Bane (Tom Hardy).
Along with Selina Kyle, a.k.a. Catwoman (Anne Hathaway), Gordon-Levitt's John Blake is one of the new characters introduced in The Dark Knight Rises. A uniformed Gotham Police officer who still believes in the Batman even after the city deemed him a criminal, Blake deduced that Bruce Wayne is the Dark Knight based on a fateful encounter the cop had with the billionaire playboy when John was still at St. Swithin's, a home for wayward boys funded by the Wayne Foundation. Blake became an ally of Batman in the fight against Bane and the League of Shadows, and he saved the life of Commissioner James Gordon (Gary Oldman), who promoted him to detective. At the end of The Dark Knight Rises, Batman saves Gotham from a nuclear weapon and seemingly perished while flying the bomb out to sea in his aircraft, the Bat. Later, it's revealed Bruce Wayne faked Batman's death so he could start a new life in Italy with Selina Kyle, finally free of the responsibility of being Batman.
The Dark Knight Rises dropped the revelation that John Blake's first name is Robin when he received an inheritance from the "late" Bruce Wayne: a duffel bag of spelunking equipment and coordinates to the secret Batcave beneath Wayne Manor. There, Blake found Batman's secret lair in a scene echoing Bruce's discovery of the bat-riddled cave in Batman Begins. The film concludes with Robin John Blake standing atop one of the pillars containing Batman's gear as it rises above the water, followed by a fade to black and the film's main titles: The Dark Knight Rises. The imagery is meant to evoke that Blake is going to be Gotham's new Dark Knight, meaning the new Batman — not that Blake will be Robin or Nightwing or a different costumed persona. (Although Blake could adopt a new costumed identity if he chooses.)
John Blake doesn't have to become Robin because he already is Robin — that's his real name, which writer Jonathan Nolan told Empire was "a wink" to the audience because "It is a little hard to imagine Robin working in that universe, so the idea had to be limited to that gag at the end." However, John Blake's backstory incorporates elements of three Robins, Dick Grayson, Jason Todd, and Tim Drake. Like Grayson and Todd, Blake is an orphan. John became a police officer just like Dick did but he also harbors lifelong anger like Jason. And, like Tim Drake, Blake figured out Bruce Wayne was Batman on his own.
Joseph Gordon-Levitt himself told Cinemablend that Blake becoming the next Batman was the idea behind The Dark Knight Rises' ending:
There’s a theme that runs through all three of [The Dark Knight] movies that begins in the first movie, runs through the second movie and it concludes in that moment where he says that Batman is more than a man, Batman is a symbol. And so to have another man other than Bruce Wayne kind of becoming Batman at the end of that trilogy, I think that’s the perfect ending to that story.
Further, Robin John Blake taking on Batman's mantle doesn't just thematically make sense, in accordance with Nolan's grand idea of Batman as a legend bigger than Bruce Wayne, it also nods to DC Comics, where Dick Grayson has assumed the role of Batman on more than one occasion. Jonathan Nolan is also right that the red, yellow, and green-garbed Robin wouldn't fit in the Dark Knight universe he and Christopher Nolan designed. John Blake later becoming Nightwing isn't out of the question but the big idea The Dark Knight Rises closed on was that Robin John Blake would take over for Batman and perpetuate the legend Bruce Wayne created, not necessarily start his own costumed identity.