The poem sets two moods against each other. In the first stanza, morning arrives pale and white, bringing with it a flood of "visions," ideas or promises that all seem like they might rescue us. The word "seeming" matters here. The speaker doesn't quite trust this brightness, and the offer of salvation feels a little too plentiful to believe.
Then comes "and yet," and the tone turns inward. The speaker is "out this night," alone with "my one head," and that single mind becomes "a thriving ship of the dead." It's a striking image: a lone person carrying a whole crowd of the departed in memory or thought, and somehow that crowd is lively, even flourishing. The title, "Endarkenment," plays on "enlightenment" and hints that real understanding may come through darkness rather than daylight. The mood is quiet, eerie, and oddly accepting. The speaker seems more at home with the dead than with the bright, easy promises of morning.