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.
How to Grow Geraniums. Depending on where or how you grow geranium plants, their needs will be somewhat different. Indoors, geraniums need lots of light for blooming but will tolerate moderate light conditions. They also need indoor temps of around 65-70 degrees F. (18-21 C.) during the day and 55 degrees F. (13 C.) at night. These plants need to be grown in well-draining potting soil as well.