Hi! A known issue - the Prevent Black Appearance for lights does not work when viewed through reflections or refractions, only for when viewed directly. The solution is to create two light sources, the one you have already to cast light into the scene (but switch it to be invisible in reflections, refractions, to the camera), and then a second light source that does not emit light, is visible to everything, and has no directionality (you can use geometry painted with a CoronaLight material too).
