blue angel vs indigo

blue angel

noun
  • A blue flame produced by lighting one's flatus. 

  • A mollusc of genus Glaucus. 

indigo

noun
  • A purplish-blue colour 

  • An indigo-colored dye obtained from certain plants (the indigo plant or woad), or a similar synthetic dye. 

  • An indigo plant, such as from species in genera Indigofera, Amorpha (false indigo), Baptisia (wild indigo), and Psorothamnus and Dalea (indigo bush). 

adj
  • having a deep purplish-blue colour 

How often have the words blue angel and indigo occurred in a corpus of books? (source: Google Ngram Viewer )