The absorbed energy is converted to heat energy. The scenario you mention with the LED is very close to the blackbody problem known as the ultraviolet catastrophe.. There is a Wikipedia article about it here. What happens in the situation you described is that the light proof box gets hotter.
Diagnosing rust on geraniums can be a challenge for growers. Rust begins with yellow spots on the underside of the leaves and can rapidly progress with brown fungal spores called pustules in the center of each spot. The pustules are raised up off the leaf surface and feel rough to the touch.
Allow the soil get dry in between each watering. Test the soil by poking your finger into the top inch. If the soil is dry all the way through the top inch, provide the flowers with enough water to dampen the soil without soaking it. 3. Fertilize your geraniums once a month using a liquid fertilizer. An abundance of fertilizer will cause your geraniums to produce many strong, healthy leaves, but it will also prevent blooms from developing and may lead to a plant with minimal flowering. You can also use a slow-release, granular fertilizer.