rue vs sing the blues

rue

noun
  • Any of various perennial shrubs of the genus Ruta, especially the herb Ruta graveolens (common rue), formerly used in medicines. 

verb
  • To repent of or regret (some past action or event); to wish that a past action or event had not taken place. 

sing the blues

How often have the words rue and sing the blues occurred in a corpus of books? (source: Google Ngram Viewer )