coal black vs jet-black

coal black

noun
  • A black colour, like that of coal; charcoal. 

adj
  • Of an intensely black colour, like that of coal. 

jet-black

noun
  • Coal-black; the blackest black. 

adj
  • Pitch-black; of the blackest black. 

How often have the words coal black and jet-black occurred in a corpus of books? (source: Google Ngram Viewer )