blackening vs denigration

blackening

noun
  • The act or process of turning (becoming) black in colour. 

  • Any of various (dark-colored) surface treatments for metal substrates, to inhibit corrosion; on steel, generally, synonym of bluing. 

  • The act of defaming or sullying. 

  • The result or an instance of such an act or process; a black stain or mark. 

denigration

noun
  • The act of making black; a blackening or defamation. 

  • An unfair criticism. 

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