Though others say the Dark Lady lacks conventional beauty, the poet insists she is his fairest jewel—her only true ugliness lies in her cruel deeds, not her dark appearance.
Physical blackness is repeatedly redeemed, but moral blackness (cruelty) cannot be. The pun on 'black' shifts from physical to moral valence.
The poem hinges on the word 'judgement' (line 12): 'Thy black is fairest in my judgement's place.' The poet claims absolute right to his own aesthetic judgment despite popular opinion. This is a rare assertion of personal taste against cultural consensus. Yet it's also a lover's rationalization: she must be beautiful because he loves her. The sonnet performs the logic of desire—constructing beauty to justify attraction rather than being drawn by pre-existing beauty.
The final couplet makes a crucial distinction: 'In nothing art thou black save in thy deeds.' Her physical blackness is innocent or even admirable; her moral blackness (cruelty, disdain, tyranny) is the real darkness. This anticipates modern vocabulary about toxic relationships: the problem isn't her body or appearance but her character. Yet the poem also suggests that these 'black deeds' are part of her appeal—her cruelty and tyranny are what make her a compelling subject.
Like defending a partner's appearance against your friends' criticism while admitting they're emotionally abusive. You separate physical beauty from moral character, insisting 'they're gorgeous but treats me terribly.' The poem conflates two kinds of darkness—physical beauty and moral corruption.