It doesn't really block any light iff installed well. Luckily, you are doing a very short greenhouse and it will be a breez to get dialed.* When it's all rolled up,* that strip down the middle will be about 6".* The clear plastic cover diffuses light and sends light in different directions.
To make a 10ft wide hoop, it takes a 20ft wide piece of clear plastic.* 6" of that will be blocked by the blackout bars when it's all rolled up and it's mounted at the highest point in the house. Doesnt matter what position the sun is, there shouldn't be a shadow casted on any part of your plant canopy at any time of day.
I have used a system just like the link you posted. A cable spool attached to the roller to pull the bar as it unrolls when you don't have gravity to pull it. I don't think it would work for up and over. It really just to move the bar horizontally. I used it to unroll an insulating blanket.