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. Eventually, it developed a second sense of "to make black" ("factory smoke denigrated the sky"), but this sense is somewhat rare in modern usage. Nowadays, of course, "denigrate" can also refer to belittling the worth or importance of someone or something.