The Blue Egg is a limited rare egg in Adopt Me! that was available at the start of the Pets Update. 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 return it to Eggburt, who would give the player an unusable Pet Egg in return; this egg 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.