About the “permanent” part:
As @kwahn stated him/herself, it was probably not really permanent, but instead “a very long time”, since it got stacked multiple times, similar to Permanent Shakes. It probably would have worn itself off, given enough time (whatever “enough” would have been).
As for “blindness shouldn’t stack”:
While I don’t have any background in “eye science” or experience with flash grenades, I think it would be possible to stack the effect (although probably not 1 grenade = 5 seconds, 2 grenades = 10 seconds …).
I’ve once got (temporary) blinded by a magnesium fire and suffered from blindness longer than a colleague who was exposed to it for a shorter period.
As for the “you would close your eyes” argument: Our lid is actually pretty weak and lets enough light pass to blind you given a strong enough flash. Covering your eyes with a body part or using a blindfold however is a different story.
Light. Sorry, you made me do it 
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Sunlight, just as @Sediment wrote, to be exact. It contains “generic” light (as in; visible wavelengths) strong enough to blind you temporarily and UV-light which will cause incurable damage to your eye if it’s exposed long enough to it.