The Blue Egg is an exclusive egg that was only available at the very start of the Pets Update in Adopt Me!. It was the very first egg along with the Pink Egg. It costed 100, and can only hatch into an uncommon Blue Dog. It is now only obtainable through trading with other players. Another similar egg that came out was the Pink Egg that could only hatch into a Pink Cat.
It is a simple egg, shaped like the Pet Egg, but it's colored a lighter blue color. The Blue Egg's shape is identical to the pet egg.