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.