The speaker feels watched and studied, but can't tell by whom. The opening line borrows the language of scientific trials, where "blind" means the participants aren't told what is being tested. Stack those layers up to "n-tuple blind" and nobody, maybe not even the experimenters, knows what is going on. The second stanza sounds like a logic idea: some truths in a system can only be proven from outside it, by something more powerful. The speaker seems to be applying that to life itself. If we can't step outside our own system, we can't prove what matters.
From there the poem slides into loss. Light becomes "an evil," a veil eaten away by moths. Rain is remembered as something that once had a body. The broken, fading lines at the end, with "st-&-ing" water and a god's image growing old, make the faith in any all-knowing watcher seem to wear thin and dissolve. The mood is anxious, cold and quietly bleak, with a dry, wary humor underneath.