blue angel vs sea swallow

blue angel

noun

  • A mollusc of genus Glaucus. 

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

sea swallow

noun

  • A mollusc of genus Glaucus. 

  • The common tern (Sterna hirundo). 

  • The storm petrel. 

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