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.
Geraniums, members of the family Geraniaceae, often appear in the landscape boasting lily-pad-like foliage and globe-shaped flower clusters atop tall stalks. Annual geraniums can be encouraged to sprout new flowers continuously if the old flowers are removed from the plant just before seeds start to form. New flowers are created by the plant in an attempt to spread its seed before its short life is over. Deadheading is a simple process, but requires a careful eye so that you remove nearly spent flowers before they go to seed.