It seems to me that the speaker of this poem is in a sense claiming authority or posession of the white trio in the first line, like finders keepers. And also, something that may have lead him to think of the "design of darkness" may be that he discovered the trio at night, or before dawn (notice that in L5 they are "ready to begin the MORNING right" and L12 "thither in the NIGHT") Physical darkness always makes things seem spiritually or phychologically dark as well.
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