I often thought that Paul came in contact with an Extraterrestrial (Angel, Christ, whatever you would like to believe) on the Road to Damascus. ET technology can blind you with Nanobots. The Nanobots search out a creatures eyes, and then embed themselves over the surface of the eye (in particular, over the pupil). The Nanobots simply cut off the light entering the eyeball, and cannot be removed without damaging the eye. But, when deactivated, they simply can be washed out and appear like fish scales.
That's my theory.
No sauce.
Just lots of pots.
Fish scales was just the easiest way to explain what happened back then, with no knowledge of nanotech.