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.
Martha Washington geraniums require six hours of sun each day. If they receive less, than that, the plants tend to get leggy and may not bloom profusely. Although the plants prefer full sun, it's best to keep them out of intense midday sun in the summer to prevent the leaves from burning.