The word blind can be used as a singular noun in the context of hunting equipment and architecture.
A duck blind or deer blind is a small camouflaged house, box, bunker, tree-mounted chair or free standing tower and is meant allow hunters to ambush prey that is lured by bait.
A blind is also a piece of material, usually cloth or plastic, set in an array across a window on a track or line that prevents light or vision from penetrating a specific area.