does Macbook Pro continue refreshing screen when brightness is set to zero?
Yes the screen keeps refreshing regardless of the brightness.
If you still have a mac with a glowing apple logo you can even see this,just dim the screen all the way and shine a light through the back of the apple logo, you can see the screen is still active in the light leaking through from the back.
Edit: since I can sense the follow-up question allready:
You can achieve what you want using one of the ways described in this article:
Stop screen rendering on macbook