The Meaning of Life, the Universe, and Everything.
I do not think it is possible with command blocks, but it may be easy with custom beacons using sethblings custom beacon filter. Then you set the blindness effect to the beacon in mc-edit, and ta-da, perfect. Just make sure the beacon in only having its effect on the part of the map you want, I think the beacon effect goes 40 blocks in all directions. Hope this helped!