Blackness rhymes with:
slackness, slackness, bleakness, darkness, frankness, harkness, hawkiness, homesickness, likeness, preakness, quickness, sickness, slackness, thickness, uniqueness, weakness
Blackness sounds like:
balchunas, balconies, balkanize, balkans, balking, belching, belgians, belgium's, bellyaching, bilking, blackening, blackens, blackmun's, blaesing, blakeney's, blasing, blazing, bleaching, bleakness, blessing, blessings, blocking, blossoming, blossoms, blowguns, bluejeans, blushing, bolognese, bolognesi, bulging, bullishness