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.
Often geraniums are grown at temperatures that are too warm. Many houses and apartments have radiators by the windows to compensate for heat loss. Thus, maintaining plants near a light source without overheating may be a problem. Avoid cold, drafty areas as well as hot, dry locations.n the home, geraniums do not require frequent fertilization. By potting young plants in a good soil mix (1 part soil, 1 sand, 1 peat), additional nutrition will not be required for two or three months. Water-soluble 20-20-20 can be used at the rate of 1 level teaspoon in 1 gallon of water.