Blackening rhymes with:
slackening, slackening, awakening, beckoning, chickening, darkening, dekoning, likening, quickening, reawakening, reckoning, sickening, slackening, thickening, weakening, wilkening
Blackening sounds like:
balchunas, balconies, balkanize, balkans, balking, belching, belgians, belgium's, bellyaching, bilking, blackens, blackmun's, blackness, blaesing, blakeney's, blasing, blazing, bleaching, bleakness, blessing, blessings, blocking, blossoming, blossoms, blowguns, bluejeans, blushing, bolognese, bolognesi, bulging, bullishness