I wish I knew exactly what about the heal-all this speaker finds so sinister. "What had that flower to do with being white?"; more like "What's the big deal about this flower's whiteness?" Is the flower also dying? "Froth" and "blight" suggest this, but that doesn't help my comprehension any. In the third quatrain, I wonder if the trouble with this plant is that it's more white than "wayside blue," and this whiteness concealed the spider, which caught and killed the moth? And therein lies the "design of darkness to appall..." The only reason I doubt this is that the capture seems to have occurred "in the night," which renders the whiteness irrelevant. Perhaps it is meant to be ironic.
Anyway, I read this as a response to writers like Emerson, who saw "design" in nature, but saw it positively. This speaker seems to say: if a 'designer' even deigns to govern something as small as this, his is a dark design. Which is more comforting-- no designer here, or an evil one? On that note, I can't help but read "the design of darkness to appall"
as the poet's capability, as well as the deity's. This is a sonnet,
after all, and a carefully constructed one, at that. I don't know if
this conflation of poet and God is intentional, but design governs in a
sonnet, right? And a sonnet is a relatively "small" poem, hey?
So, we have here either a sinister God or a sinister poet, and in either case, the spider and moth are but ignorant, helpless pawns. In the second reading, does this speak to the reader, as well? Is this an omen that Frost is sending an albino Aragog after me? That was a joke. I think the second answer is in line with pragmatism, and I certainly prefer it to the first idea of a some evil, mocking deity.