there are commercial blackout systems availble for chrysamthemum growers.
in 1988 and 89 i blacked my 60 ft polytunnel crop from 1st july until 1st of sept.
i welded up a wheel system and staked rails into the ground .then used the black white mushroom tunnel cover to cover the frame and blacked out the crop by rolling the whole polytunnell over them 6pm until dark then rolling it back off.the plants where in pots and had been grown for 3 mnths in a normal poly tunnel .
definatley worth looking at chrysanthemum black out systems .
It is simple. Build polly tunnel that is 12 wide and 7.5 tall it can be as long as you want lets say 80 feet. Now take a 20 f X 100 blk plastic can easly be pulled over the hoop house.This is the easiest light dep going. Use 6 mill plastic you should be able to get it form any hard wear store. . You may need 2 sheets to achieve total blk out. A house like this can be built in one day with a 9 pound hammer, 20 pieces of 3 foot rebar, and 12 20 foot pvc a least 1 inch in diameter. You need some random wood but easy as can be. Go out in to the ag land in your area and see what the food farmers use.