I don't don't know what might make them glow, as you said it could be the minerals in the soil that they have taken in, but I do know that using a black light will cause some mutations in the peppers that you are growing, most likely in the young seedlings and or possibly in the seeds when your plants start making pods. I had this happen several years ago when I was trying to make some mutations, most of which were weak and not worth growing, as some of the peppers genes were damaged or missing.
George W. Z5-6 Mo.

This the Chernobyl Pepper? lol

There are many materials that fluoresce under black light.

It's only unusual if a pepper grown in sunlight doesn't do the same.

alien DNA!!!!???

oh no oh no alien peppers have invaded our world!!!

no seriously i think its the minerals, peppers do contain and supply some phosphorus and other minerals, and yes they do pick them up from the soil

hope this helps thanks your friend Joe

Not black light but Pepper X Rays.
Saw some that were colored too.
Really cool looking.

Here is a link that might be useful: X Rays

Minerals yes. But, that is one really neat looking photo!