black tar vs cocaine

black tar

noun
  • An illicit drug formed from the incomplete acetylation of morphine, containing a variable percentage of heroin. 

cocaine

noun
  • Any derivative of cocaine. 

  • An addictive drug derived from coca (Erythroxylum) or prepared synthetically, used sometimes medicinally as a local anesthetic and, often illegally, as a stimulant. 

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