dullness has 2 distinct synonyms spread across 2 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its 2 recorded opposites are brightness, sharpness.
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.
the quality of lacking interestingness
“the stories were of a dullness to bring a buffalo to its knees”
a lack of visual brightness
“the brightness of the orange sky was reflected in the dullness of the orange sea”
lack of sensibility
“there was a dullness in his heart”
without sharpness or clearness of edge or point
“the dullness of the pencil made his writing illegible”
Synonyms, antonyms and the broader and narrower relations for dullness come from Princeton University’s WordNet. Full sources and licences.