Cara Dune calls the New Republic pilot Carson Teva "Blue" in The Mandalorian season 2, episode 4, "Chapter 12: The Siege", but the show doesn't give a clear reason as to why. Very little is known about Cara Dune's past, but what has been revealed in seasons 1 and 2 indicate that she's from Alderaan and was a shock trooper in the Rebellion - a good one at that. She developed quite a reputation among veterans in the Alliance, so much so that she's still noted for her service years after the Galactic Civil War ended.
Something happened, though, and Cara Dune wound up leaving the Rebellion to spend the rest of her days in the Outer Rim. But after fighting alongside Din Djarin and Greef Karga in The Mandalorian season 1, she's now become a marshal on Nevarro in season 2, fighting criminals and doing her best to rid the planet of the Empire. It's there that the New Republic eventually catches up to her, after they destroyed an Imperial science facility outside of town. One of those pilots is Carson Teva, who attempts to have Cara Dune rejoin the Rebellion.
When Carson Teva finishes up taking Greef Karga's statement about the Imperial incident, he meets Cara Dune outside and says, "According to records, you're quite a soldier. We could really use you." Cara Dune then replies, "I'm not a 'joiner,' Blue." The nickname is odd since it hasn't been used before, and on the surface, it's possible she's referring to Carson Teva being part of Blue Squadron. However, he's not wearing the Blue Squadron flight suit (as seen in Rogue One: A Star Wars Story). Instead, the Blue nickname likely refers to his helmet being all blue with the Alliance insignia in gold. Compared to the Rebel helmets from the original trilogy, the all-blue one is quite unique.
Another possible explanation, though unlikely, is that Cara Dune calls Carson Teva "Blue" because of him acting like a police officer. In "The Passenger", he and Trapper Wolf stopped Din Djarin on a routine patrol, requesting his ship's information. And now, he appears on Nevarro to take a statement about the Empire's presence in the system. All of it points to him being a sort of police officer in the galaxy, which is something Cara Dune referred to in her season 1 debut. She once told Mando that after the Empire fell, the Rebels became peacekeepers, which wasn't what she signed up for. As someone who lost everyone when Alderaan was destroyed, she wanted to take down the Empire, not restore peace in the galaxy.
Given that the Imperial remnants aren't going away and The Mandalorian is laying the foundation for the First Order's arrival, it's unlikely this is the last time viewers will see Carson Teva. He gave Cara Dune a Republic medallion of sorts, which may either officially deputize her as a New Republic marshal or encourage her to continue the fight against the Empire. Either way, The Mandalorian is far from done with its Imperial story.