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”
Synonyms, antonyms and the broader and narrower relations for dullness come from Princeton University’s WordNet. Full sources and licences.
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”