There were once many magic mirrors that could create foal clones of any pony who looked within, but most were lost to time after Dream Valley was abandoned. One cracked mirror, however, remained, and it was discovered that it could create a foal of any two ponies who aligned themselves in just the right way. It was this mirror that Applejack and Rainbow Dash used to produce their foals when the time came.

Blue is Applejack and Rainbow Dash's youngest foal, and has been fairly spoiled most her young life. She loves to play with dolls, dress up, and other typically girly things, much to her mothers' and sisters' dismay. She can be a bit snobby and stuck up to other foals, though she rarely is an all-out bully.