I can't be certain, but I concluded that the replacements(?) I noticed had never had any blind in them because there was no channel in which the blind could slide (up
or down).
I remember reading two or three years ago that as well as removing trays from seatbacks, MO'L was thinking about doing without window blinds too ... maybe Boeing made a small stock 'just in case' and when it was decided to drop the idea, they put them on a shelf in Seattle until MO'L needed some quick replacements? (if so serves him right!)
Seriously though, have these things (overwing exits, not blinds!) had to be replaced with any significant frequency?