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.
Bacteria, fungal pathogens and insects can cause fruit trees to develop black leaves. Bacterial spot is the result of the bacterium Zanthomonas pruni attacking fruit trees. It typically presents itself as discolored spots on the foliage that begin to blacken and enlarge and can engulf the entire leaf.