Blue axolotls are by far the least common color in Minecraft since if you don't find one in the wild, the only way to get one "naturally" is by breeding two other colors and hoping for the blue mutation to appear. 99.917% of the time, a baby axolotl will match one of its parents; random occurrences of blue make up the remaining 0.083%. Because they're so hard to get, many players use console commands to spawn blue axolotls. Once you have one, however, your odds of getting another through breeding increase immensely.