What is a "Blue Sun"?

site: www.xrefer.com/entry/612319 A blue moon or blue sun is partly abscured by large particles in the atmosphere from dust storms, forest fires or great volcanic explosions giving the sun and moon a bluish glow. More commonly seen in China than anywhere else.
Expression 'once in a blue moon' means never.

Volcanic eruptions and forest fires are known to cause a blue-looking sun or moon. The particles in the dust or smoke scatter the red end of the spectrum, leaving an excess of blue.

After the eruption of Krakatoa, an island volcano in Indonesia, in 1883, blue moons and suns were widely observed.

Astronomers have two definitions for the term blue moon. One definition refers to when two full moons occur in one month. This happened twice in 1999. An older definition of blue moon is when four full moons occur in one season. The 500-year-old expression "once in a blue moon" is thought to pre-date the calendar meanings of the term.

http://archives.cbc.ca/IDC-1-70-327-1733-11/that_was_then/disasters_tragedies/blue_sun