I think the answer is
It's 12 o'clock!
As per their long-time ritual
Clocks are very related to time!
The Black Knights sat in a circle.
The Black Knights are the numerals around the face of the clock.
The captain sat at the head,
and to his right the colonel.
A runt sat left of the captain,
half the size of the colonel.
The captain is '12', the colonel is '11', and the runt is '1' (which is half of '11' by physical size).
The left and right might seem backwards, but it must be from the point of view of the captain.
A bold accuser stood in the center,
and quietly his votes were cast.
The accuser is the hands of the clock pointing at the Knights.
When the captain was accused, not once, but twice,
the silence did not last.
When both hands of the clock point towards the captain, the clock rings the chime.