I am using blackouts for my room, as curtains... It gets too bright in the summer mornings
It is a considerably heavy fabric, velcro might not carry it for long especially if you plan to cover a large area.
Even though it blocks light perfectly, it is not %100 light proof, but 2 or 3 layers of blackout would be 100% light proof IMO.
I believe it would be a good idea for portable solution, but extra layers would increase the cost.