"Denigrate" means to tarnish a person's reputation. It has nothing to do with race, as far as I know.

1. To attack the character or reputation of; speak ill of; defame. 2. To disparage; belittle: The critics have denigrated our efforts.
ETYMOLOGY:        Latin dnigrre, dnigrt-, to blacken, defame : d-, de- + niger, nigr-, black; see nekw-t- in Appendix I.

The fact that it has a root in common with the color is not to be taken literally, as reputations do not have colors as far as I know.

A