Equip the perk Flashbang or Blast Mine, and simply blind a killer with either of these perks. The blinds resulting from the explosion will not count towards Blindsided, a challenge in the current tome.
Did you check if the killer had Lightborn?
I got the score event for Killer Blinded, so I don't think he had Lightborn, but good catch!
I (as a killer with Lightborn at the time) did get flashbang stunned briefly, and at the end of the match I let them get flashlight rescues (they won anyway) ... the flashlight will eventually quickly narrow and widen (so long, it likely can't be done without the killer pretty much giving you the time to do it without pressure), and allowed the survivor to escape without a regular blind on me, so technically possible to get the score event through lightborn, but not get the intended blind.
If you're able, try a few of these, especially Blast Mine again, and check the perks postgame. If the killer does not have any perk or add-on to prevent being blinded, and still received no progress, please post an update!
I'll see if I can do that today, thanks!
E: Apparently already did this challenge; any other way of testing this out?