Given the spooky nature of Fallout, I think it would be really fun to have a DLC where you have a quest to gain the ability to make your pipboy use a blacklight and following quests where you use the black-light from your pipboy to uncover messages around the city for hints/tips or a whole quest. There could even be enemies at night that are hard to see unless you use a blacklight to show their bodies(which are painted in cool designs).

Could even incorporate this into a children of atom quest line.

Could possibly be added to settlement building where you paint messages on walls and is only illuminated when you turn on a blacklight switch in the house. Make messages on signs that only show up at night.

Just a thought......