We know that appearances are going to be important in "She Walks in Beauty" from line 1 – after all, the fourth word of the poem is "beauty." The entire poem is one long description of a woman's beauty. How many different ways can the poet come up with to say, "she is so gorgeous"? Quite a few, as it turns out. But not all of them are conventional, so watch out.
By emphasizing that the woman's beauty really couldn't be any darker without throwing off the delicate balance, Byron might be tacitly acknowledging that he's going against the conventional standards of beauty.
In "She Walks in Beauty," Byron suggests that real beauty is only achieved through a harmonious balance of apparently irreconcilable opposites.
We're talking about the unnamed lady's principles here, not the speaker's. We're guessing – partly based on the poem itself, and partly based on Byron's reputation – that, given the opportunity, he'd happily seduce her. But this particular woman would have none of that. We're told repeatedly that she's pure and innocent, and that's part of why she's so gorgeous.
The speaker opens the poem with a description of the woman's beauty, and ends it with praise of her virtuous "innocen[ce]" in order to emphasize that true beauty requires a balance of good principles as well as physical beauty.
The speaker's emphasis on the woman's principles, supposedly in praise of her beauty, suggests that he wishes to dodge suspicions that his attraction to her is less than innocent.
"She Walks in Beauty" is completely focused on one woman. But, as you may have noticed, the woman doesn't ever to get speak for herself. Instead, she is totally objectified by the speaker. He actually breaks down her appearance and focuses on different parts of her, from her hair, eyes, and skin to the way she walks. He even says he can guess what she's thinking based on her "smiles" and her blushes!
In "She Walks in Beauty," Byron borrows from a long tradition of poetry that praises a woman's beauty by breaking her down into her component parts. This approach effectively objectifies and silences the unnamed woman.
Many feminist critics have criticized "She Walks in Beauty" for its apparent objectification of the unnamed woman. However, Byron breaks from tradition by acknowledging that the woman has "thoughts" and an inner life that he cannot access.
Because of the way the unnamed woman in "She Walks in Beauty" is described, the speaker almost seems to be worshipping her. He idolizes her beauty and compares it to things that are so vast and universal that her beauty seems almost supernatural.
By comparing the unnamed woman to abstract and intangible ideas, the speaker effectively idolizes and dehumanizes her, turning her beauty into something almost superhuman.