"She Walks in Beauty" is an artistic appreciation of the internal as well as the external beauty of a woman with an emphasis on how the duality of loveliness "meets" in her person.  The controlling metaphor for this duality is in the comparison of the two facets of the feminine object of appreciation, expressed with the employment of light and dark imagery.

In each stanza, there is the comparison of physical characterization with the moral depiction.  For instance, the speaker says that the lady walks like the night of starry skies

And all that's best of dark and bright

Meet in her aspect and her eyes

The "eyes" are an apt feature to illustrate both physical and inner beauty as they are the feature that best reveals the heart of a person.  Again, in the second stanza, the light/dark imagery pervades as a descriptor of the dual characterization as a "nameless grace" is apparent in her "raven tresses," her inner being.  Or, they lighten her face where they are more easily apparent.

Finally, in the final stanza Byron's speaker expresses fully the idea that the purity and beauty of soul apparent on the woman's features emanate from within, giving a glow of "a mind at peace" and a "heart whose love is innocent." Indeed, throughout Byron's poem, the lady is the object of the dual appreciation of physical and spiritual beauty, an inner beauty that even lends beauty to the physical features of the woman.