Watching lightning from too close of a distance is supposed to be bad for your eyes.
But if you were close enough to the lightning to have the lightning cause blindness you'd likely get struck by that lightning as well.
I've observed and watched lightning for years from a safe distance without any eye protection and never has it harmed my eyesight as a result of watching the lightning.
What I've heard about watching lightning is that when you watch a flash of lightning your visual nerves can be damaged as a sudden result in the change of light coming into your eyes.
However the worst that would happen is you experience seeing yellow colors or have temporary blindness.
You're more likely to get shocked as a result of the lightning than to actually go blind when watching lightning.
If you're worried about going blind from the lightning then you could wear some sunglasses while watching lightning but it's not required.