If you "denigrate" someone, you attempt to blacken their reputation. It makes sense, therefore, that "denigrate" can be traced back to the Latin verb denigrare, meaning "to blacken." When "denigrate" was first used in English in the 16th century, it meant to cast aspersions on someone's character or reputation.

denigrate [ den -i-greyt ] Phonetic (Standard) IPA verb (used with object), den·i·grat·ed, den·i·grat·ing. to speak damagingly of; criticize in a derogatory manner; sully; defame: to denigrate someone's character.