The game currently has five different colors of the little in-game amphibians: cyan, pink, gold, brown, and blue. Blue is super rare, and there’s only a 1 in 1,200 chance that the baby axolotl you’ve coaxed out of your tropical fish-induced love frenzy will be blue. Usually, the tiny beings will take on one of the colors of their parents; however, you might get lucky and find yourself with a blue one.
It should be noted that there really isn’t anything special about blue axolotls that sets them apart from their pink, brown, gold, and cyan counterparts, they act exactly the same. Still kinda cool though, like nabbing a shiny Pidgey in Pokémon.