Antonyms for Sharpen (opposite meaning)
WordNet sense 1 (make (images or sounds) soft or softer):
soften
WordNet sense 2 (make dull or blunt):
blunt, dull
WordNet sense 3 (lower the pitch of (musical notes)):
flatten
WordNet sense 4 (make dim or indistinct):
blear, blur
From the ODE community, based on WordNet
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