Black Rainbow has a duration of roughly 12 minutes and is composed in two movements. The music was inspired by the design of the Pageant Amphitheater at the MacDowell Colony in Peterborough, New Hampshire, which Bray described as a "stunning space" with "intense, powerful energy." She wrote in the score program notes:
The work is an expression of imagined scenes played out here. A rainbow seen at night can be referred to as a black rainbow; difficult to see, the colors are often hidden; a metaphor for something sought after but impossible to attain, an alluring ongoing search.[1]
She also described the nature of the two movements, adding, "The first movement is dark and ritualistic. It’s mid summer and the air is tight, uncomfortable. The light grey, purple almost. The second is fleeting, sensual; time is suspended, a dream-like state."[1]
The work is scored for an orchestra comprising three flutes, three oboes, three clarinets, three bassoons, four horns, three trumpets, three trombones, tuba, timpani, three percussionists, harp, and strings.[1]