blue angel vs sky blue

blue angel

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

  • A mollusc of genus Glaucus. 

sky blue

noun
  • A vibrant light blue colour, like that of the sky on a fine day. 

adj
  • Of a vibrant light blue colour, like that of the sky on a fine day. 

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