Blue in vertebrates is nearly always structural. Layers of guanine platelets in iridophores interfere with incoming light and reflect a narrow band back, which is why a blue feather crushed to powder turns brown. The colour lives in the geometry, not in a molecule.
Mandarinfish and the closely related psychedelic mandarin, Synchiropus picturatus, break that rule. Their skin contains cyanophores holding a genuine blue pigment in membrane-bound organelles. The pigment has been characterised as distinct from the carotenoid and pteridine chemistries that produce reds and yellows in other chromatophores, and its cells respond to nervous control like other pigment cells.
The mandarinfish also lacks scales. In their place it secretes a thick, foul-smelling mucus that is thought to deter predators and may protect against infection, and this coating is one reason the species is difficult to keep alive in aquaria: it also refuses most prepared foods and needs a constant supply of live copepods.