Spiritual, of course, is an old word, but its first general application to Negro religious song probably occurred after the tours by Fisk and Hampton singers, and may have resulted from an article in Harpers Mag. 1866, May 775/1. A brief entry in the OED under Negro 7, spiritual reads "Maum Rima flavored all her dishes with these 'spirituals' as they are called among the Negroes." If someone has access to a library with a Harpers Mag. file, this article would be worth looking up.