The game has truly been a way for lifelong trainers to take their Pokémon hunts into the real world, connecting with friends internationally while they play. But if you’re still new to the game, there are some things that aren’t explained to players right out of the gate, leaving some with questions.
For example, when scrolling through your collection of Pokémon, you may find that some of them have a blue background behind them, making them stand out from the others. What does the blue background mean?
Source: https://www.distractify.com/p/what-does-blue-background-mean-pokemon-go