That colloquial expression gave American composer Sean Shepherd the title and structure of “Blue Blazes,” a piece that the National Symphony Orchestra premieres this week. This is Shepherd’s third appearance with the orchestra since 2009, when he was one of fifteen composers invited to be part of the CrossCurrents Contemporary Music Week. That same year, British conductor Oliver Knussen premiered his “Wanderlust” with the Cleveland Orchestra and conducted it again with the NSO earlier this season. “Blue Blazes” is one of the pieces Maestro Eschenbach has chosen to take along on NSO’s Latin American tour. “I knew that the other works on the program, Beethoven’s Symphony No. 7 and the Suite from ‘Der Rosenkavalier,’ are right in the center of what Eschenbach does, so I thought this would be a good opening piece, a brief statement,” Shepherd said. “Months before writing any notes, I chose the title. Going through the process of deconstruction, I thought about the concept of ‘blaze’ and other meanings of ‘blue.’