Darkest Night Batman is a hero that may or may not have existed prior to the public release of the game. He seems to appear in-game in the Arena picture on the home screen, and an image containing a Bronze card and a Gold card variant of "Darkest Knight Batman" was one of the game's "images" in the Google Play store.
It's notable that in the image in the play store, there is both a bronze variant and a gold variant. At whatever stage of planning or development the image seems to come from, there was likely a different collection and promotion system. There are no Bronze rarity cards in the game. He may be based on the comic Batman: In Darkest Knight, but there is no indication of him as a Green Lantern.