The poem is a pair of small images that mirror each other. In the first, someone opens a safe just a crack, hoping to catch sight of night inside it. A safe is a place for keeping valuable things, so the idea that it might hold darkness is both odd and a little funny. Darkness is what any closed box contains, yet the speaker treats it like a treasure worth checking on.
The second part turns to miners who dig and find no ore. The title, "Robbed out," suggests a mine or vault that has already been emptied, and the poem seems to say that what's left is only absence. The word "poor" and the phrase "troubled to find" give the miners a gentle, almost sympathetic touch.
The mood is quiet, wry, and a bit wistful. The speaker doesn't sound angry about the emptiness. Instead there's a soft humor in searching so carefully for something that was never really there, or is already gone.