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.
The last geranium flowers of winter. I think the difference is based on the flowers. The white and red geranium still had flowers now, while the red one stopped blooming some time ago. The stress of having to survive in the cold plus showing that nice display of flowers is what made the white and red geranium more susceptible to getting sick.