"Straw bluing" would be an incorrect term, since it is not blue. It is a yellowish or golden ("straw") color which results from the heat treatment of parts. Originally a simple byproduct of the the heating, it was later done for decorative purposes by careful control of the temperature. It is not very permanent, but looks great when new. A prime example is the Luger; Pre-WWII era Lugers have the trigger, safety, magazine catch, takedown latch and ejector straw colored. Combined with the soft rust blue also used in that era, the result is a beautiful firearm.