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.
Also known as the tobacco budworm, the geranium budworm is a caterpillar that tunnels into unopened buds and eats them from the inside out. Then when they open, the flower petals are riddled with holes. To prevent the spread of the worms into healthy buds remove any buds showing tiny holes and nearby brown specks.