If I pass during some nocturnal blackness (9)
The word "pass" here could be a pun on the word "past" – and when someone "passes" (i.e., dies), he switches from the present to the past, so the pun could make sense.
Will this thought rise on those who will meet my face no more (15)
The "thought" that "rises" is a memory of the speaker, but it's almost an unconscious memory – the neighbors "who will meet [his] face no more" aren't expecting to recall the speaker, but the beauty of the stars makes the "thought rise" in their minds without them being aware of it.
"He hears it not now, but used to notice such things" (20)
This sentence strongly juxtaposes the present and the past: it starts with the present tense, and then shuts the speaker out of the present and puts him in the past as firmly as the "Present" slammed that "postern" door on him in the first line: the speaker "hears it not now, but used to notice such things."