I'm guessing you mean the Western Blue Devil, Paraplesiops meleagris? Here is an excerpt about them from my Advanced Marine Aquarium Techniques book:
Western Blue Devil (Blue devilfish)
Paraplesiops meleagris
Maximum size: 35 cm
Habitat: Caves and ledges
Range: Southwestern Australia
This is the “holy grail†of fish from this region â€"œ at least for those who have seen color photographs of an adult specimen. Bright blue with even brighter light blue spots makes them one of the most distinctively colored fish available. Aquarists should be cautioned that it is only the older adults that are this colorful, younger fish are a blue-gray with a smattering of blue spots. In addition, their color is most intense when illuminated by a camera flash. When lit normally, with aquarium lights from above, they appear bluish gray. Growing to a length of around 14â€Â, they are shy and retiring at first, but soon begin spending time out in the open. They are compatible with other fish too large to be eaten, and with their relatively small mouths, they are better suited for feeding on krill, mysids and chopped seafoods. They may be territorial, so be cautious if attempting to keep more than one in an aquarium. The drably colored juveniles are often sold in the aquarium trade as “Allison’s devilfish†or “blue longfin grouperâ€Â.
Jay Hemdal