WordNet records no synonym and no antonym for black hills. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
mountains in southwestern South Dakota and northeastern Wyoming; sacred to the Sioux (whites settling in the Black Hills led to the Battle of Little Bighorn); site of Mount Rushmore
mountains in western South Dakota and northeastern Wyoming
Synonyms, antonyms and the broader and narrower relations for black hills come from Princeton University’s WordNet. Full sources and licences.