dark o'clock
When it's so late at night or early in the morning that it's dark out.
"Man, I stayed at work til dark o'clock yesterday" or "I woke up and it was still dark o'clock"
dark o'clock in the morning
The hours between midnight and dawn, when it's morning but it's totally dark and no one's up unless they absolutely have to be.
Go to bed early, hon. We have to catch the bus at dark o'clock in the morning to get there on time.