until after in the temporal sense wants to be complemented by a phrase that can be understood as establishing a sequence of events.
We didn't reach home until after {the baby had fallen asleep}.OK
We didn't reach home until after {the baby was sleepy}.maybe
We didn't reach home until after {sunset}.OK
We didn't reach home until after {night} not OK.
The lawyer would not comment until after {the trial}. OK
The listener will try to "make sense" of the statement. The listener could suppose that you meant "until after the baby had become sleepy", that is, the baby was not sleepy at first and then started to get sleepy. That's the only way that utterance makes sense for establishing a sequence of events. So, with "after the trial", why can't we interpret "after the trial" to mean "until after the trial had begun"? If language were utterly divorced from reality, we could do so; but we also need to bring knowledge of the world to our understanding of an utterance: lawyers typically do not want to jeapordize an ongoing trial by making comments about it while it is underway.
But how about "after night"? Why can't we interpret that to mean "until after night is over" just as we do with "after the trial"? We can indeed interpret "after night" in that manner but it is not how native speakers would express that idea, "after night is over". Native speakers would say "until morning", not "until after night". There's no way to know that, of course, except by much exposure to native speakers. And if your meaning is "until after it had become dark", you could say:
We didn't reach home until after night had fallen.
We didn't reach home until after dark.
Many set phrases like "after dark" don't seem to follow the general rule. The only way to learn such idioms is through exposure to native speakers.