These reflective surfaces are attached to arms that disappear when they’re not needed but the fact that something like that is necessary, it would seem that making blaster fire appear as part of a stage show or parade would be difficult without making the larger apparatus visible.
The technology, as uncovered by WDW News Today, is actually fairly simple and just uses a series of reflective surfaces to show off the blaster shot in motion.
In a dark ride like the Millennium Falcon flight or the First Order “Battle Escape” attractions, which we know are on the way, the arms can be better hidden and they won’t need to move around.