Short for "Retroactive Commentary," Retcom is derived from retcon
, or "retroactive continuity." Retcom refers to any situation where a movie, book, or other form of entertainment was initially made purely for entertainment purposes, and social commentary has since been added by the producers or audience.
Critical review and some of Romero's later statements aside, Night of the Living Dead wasn't meant to have any sort of major commentary on race relations - it just happened that the friend Romero picked to play Ben was black, it wasn't in the script. It's pure retcom.