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.
Sow seed geraniums in well-drained soil with a pH of 6.0 to 6.2. The seeds do not need light to germinate, but a thin layer of vermiculite helps retain moisture. Seed geraniums also benefit from light fertilization one to three days after sowing. Sow seeds indoors about eight to 10 weeks before moving them outdoors with other warm-weather annuals.