black tea vs herbal tea

black tea

noun
  • Tea leaves which have been "fermented" (actually, oxidized) before drying to enhance their flavour (cf. green tea). 

  • A cup of tea served without milk. 

herbal tea

noun
  • A beverage made by infusing the leaves of various herbs, generally not including true tea (Camellia sinensis). 

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