Background in the dark: Anna Sewell’s 1877 horse story got its title from this poem by Edward Herbert which was first published in 1665. As you see, its subject is the philosophical beauty of the idea of blackness: the one reality (you could call it death) that remains after color (you could call that life) goes dark.
A historical technicality is that in the seventeenth century the word inaccessible was pronounced “in-AC-sess-EEBLE,” as if in French. In the last two lines, “alone our darkness” means “only our ignorance.”
And a Metaphysical witticism is that although this sonnet’s prosody follows the form’s conventional division into two verse paragraphs rhyming in an 8 + 6 pattern, the idea expressed by the paragraphs is inverted, black to white and white to black, into a negative whose dimensions are 6 + 8. Read that way, lines 1 through 6 say, “Black beauty,” and lines 7 through 14 complete the predicate by saying, “what you are is the one eternal thing.”