There are a few species where the axanthic form is actually blue ruling out xanthins as the pigment responsible for blue. The most notable example is New England Strain Brooksi that are bluish. I believe there is also garters, plains infact, that the anery and axanthic morphs are blue.
Check out this axanthic Rein snake. Normaly green in colour, it is now blue.
Iridiophores are responsible for a wide range of colours, including blue.