I see ye visibly, and now believe That he, the Supreme Good, to whom all things ill Are but as slavish officers of vengeance, Would send a glistering guardian, if need were To keep my life and honor unassailed. Was I deceived, or did a sable cloud Turn forth her silver lining on the night? I did not err; there does a sable cloud Turn forth her silver lining on the night, And casts a gleam over this tufted grove.[3][4]
It is a metaphor referring to the silvery, shining edges of a cloud backlit by the Sun or the Moon.