Now that the speaker has established the idea that dark features are really the only true beauty, he then moves specifically to his lady in the sestet (the last six lines). The speaker emphasizes the darkness of the lady's dark features: her eyebrows (and hair) and her eyes are "raven black," they are extremely black. The speaker then adds a metaphor to describe the appearance of her eyes. He says they look like "mourners," like individuals dressed in black clothing appropriate for a funeral. They mourn or feel pity for those who are beautiful even though they do not have "fair" (or light) features. Such dark beauties deserve pity, the speaker probably feels, because they have been slighted or undervalued by others for so many years. Once again the poet uses personification to emphasize his point. The appearance of such dark beauties slanders or hurts the reputation of Creation, who is responsible for the false value or esteem (or worth) given to fair (or light-featured) women. In other words, such creatures (fair women, who are products of creation) do not deserve the value that people or society gives to them.
In the couplet, the final two lines of the poem, the poet concludes that even though the eyes of his lady are mourning, they make her woe or sadness seem even more beautiful. Everyone who looks at her dark eyes agrees that the Dark Lady is the very picture of beauty.