Darkness usually frightens people, but here the speaker finds it oddly gentle. Instead of fear, she feels relief from despair, as if the night could cover her scars and hide a wound she can't otherwise escape. The dark becomes a kind of refuge, a place to step away from reality for a while.
What makes this comfort possible is a sense of presence. The speaker feels that someone she loves is somehow there in the shadows and the air, listening to her words. The poem suggests this person is no longer physically with her, since it speaks of love lasting "even after this world." Her grief is real, but it is softened by the memory of being called a "forever friend."
The mood is quiet, tender and a little sad, yet it ends on reassurance. Loss hurts, the speaker admits, but the bond she shares feels strong enough to outlast it.