A dark blue paste used to assist in the mating of two or more components. The paste is rubbed onto a reference surface and then the workpiece is rubbed against the coated reference piece. The transfer (by contact) of the pigment indicates the position of high spots on the workpiece or conversely highlights low points. This inspection method can be used to test the flatness of surfaces, the trueness of bearing assemblies, and the quality of gear contact.
See also Bluing.