Secret blue (Real name is Arctic Blue) is the kind of blue which is not shown on the colors choices of the your character.
How to get secret blue[]
Click on your animal picture. Choose the fur tab, eye tab, or pattern tab. Do you see that purple color in the down right corner? Click the edge of the purple color (Not between the purple and turquoise). Ta-da!!! You got the secret blue! Just keep trying if it doesn't work!!!