black tar vs heroin

black tar

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

heroin

noun
  • A powerful and addictive drug derived from opium producing intense euphoria. Classed as an illegal narcotic in most of the world. 

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