The Blue Egg is an egg that was available at the very start of the Pets Update in Adopt Me!. It was the first pet egg and was obtainable from the Easter 2019 update through a secret quest line. Players were tasked to collect a Broken Egg from the top of the Sky Castle, and then to return it to Eggburt who would give the player an unusable Pet Egg in return, which later transformed into a Blue Egg when pets were released. Blue Eggs can only hatch into an uncommon Blue Dog. This egg is now only obtainable through trading.
Another similar egg that was released into Adopt Me! around the same time was the Pink Egg, which could only hatch into a Pink Cat, although this egg is not to be confused with how players could obtain the Blue Egg.