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.
Use a pair of hand clippers to cut the foliage back to the base of the geranium, leaving any fresh green growth in place. Prune perennial geraniums 2 or 3 inches above soil level after it dies back, which can occur anywhere from late summer to early winter, depending on your climate and variety.