P. coeruleolineatus has an elongated body that grows to a length of 8.5 to 10 cm. Although colours vary, it is generally has a black or brown body, with two dark stripes behind the eye. The dorsal spines are tipped with orange or red, bordered below with a line white. It has a bluish stripe running along the basal part of the dorsal fin.
This is a very common yet secretive species. It lives in shallow, outer-reef areas at depths to 15 m. During the day, it usually remains under coral and stones in lagoons and in flood basins. It may appear when disturbing rubble in pools. It emerges at night, venturing into the open to feed on fish, gastropods, and small crustaceans.