I agree with the suggestion for the "Command" adhesive products. My daugher has had great success with them--and each package will tell you how many pounds that particular hook (or clip, etc.) will hold. She swears by them!
How about sticky backed velcro? Though, I too, have used the 3M products and think they are really great!
I would try 3M Command Adhesive Mounting Products as a way to secure the chiffon to the wall. They also have decorating clips which would allow you to string lights and poster strips for... well... posters! Their website suggests that they will work on Cinder Block without damaging the surface.
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