Evening is settling in, and the speaker welcomes it. The poem opens with blue light, then notes that the sun's rays are gone and clouds have swallowed the sky. Instead of dreading this, the speaker breathes the coming night in, almost like taking a deep, calming breath.
The key shift comes in the middle. The "dark tones" don't add to the speaker's troubles; they overtake them, so the darkness works like a blanket over their woes. The comparison to an "aesthetic photo" where colors "go awry" suggests the odd, over-filtered look of dusk, beautiful precisely because it's slightly off. The speaker admits to being surprised that there's no "dark demise" here, no collapse into gloom.
The closing line settles it: they sit in peace and don't miss the sunny skies. The mood is quiet, reflective, and gently content, finding comfort in the dim end of the day rather than longing for brightness.