Blue demon was a pejorative term used by Tellarites to describe Andorians.
In 2154, Tellarite Ambassador Gral recalled the last time he was in Andorian space was during his race's conflict with them, where he had participated in military action against them aimed at, "driving those blue demons back into their territory" from aboard the command deck of a Tellarite cruiser. (ENT: "Babel One")