This is an example of blue coated aerospace hardware we supply, with a 1/4 diameter shoulder screw in alloy steel coated with Alumazite KL-55 blue finish.  The blue coating was intended to allow an robotic color coded system to recognize the screw and yet maintain its integrity in environments up to 600 degrees F.  The coating is also intended to limit any risk of off-gassing and usability of the hex socket drive.  Only the head is coated to avoid the complications of having any coating on the shoulder and threaded surfaces.