The poet praises his mistress's black eyes as surpassingly beautiful—they become her as mourning clothes suit the dawn, and he wishes her heart would mourn his suffering as much as her eyes suggest.
The sonnet argues that the Dark Lady's black eyes 'suit' her perfectly—they are becoming not despite their darkness but because they express mourning. This is a radically modern aesthetic: beauty consists in authenticity of feeling, not in conformity to an ideal. Her eyes are beautiful because they truthfully register her emotional state. This contrasts with the artificial beauty of cosmetics attacked in Sonnet 127.
The poem's request is subtle: let your heart mourn for me as your eyes suggest you understand pain. The poet is asking the Dark Lady to extend her apparent empathy into action. The beauty of her mournful eyes is incomplete unless it motivates compassion. This reveals the sonnet's actual negotiation: admiring her even as asking her to feel differently, to care for him. The poem flatters and petitions simultaneously.
Like finding someone's vulnerability and slight sadness more attractive than confidence. The poem romanticizes melancholy: her eyes suggest she understands pain (even if she causes his), and that understanding is more beautiful than any smile. It's the appeal of shared emotional damage.