folk song vs negro spiritual

folk song

noun
  • A song, in any folk style, originating among ordinary people, usually non-professional musicians, and handed down by oral tradition. 

  • folk music 

  • A song in the style of folk music, a modern genre of popular music. 

negro spiritual

noun
  • A genre of African American song, usually with a Christian text, and sung a cappella with no harmony; it developed into gospel music. 

How often have the words folk song and negro spiritual occurred in a corpus of books? (source: Google Ngram Viewer )