I've never used a dehydrator, but the tomatoes I've dried in the sun occasionally turned black, for one of two different reasons. One was, when we dried tomatoes behind glass in early autumn when the sun was still very hot, they sometimes literally burned. If they were just dark brown, they were yummy, but if they got actually black, they tasted just burnt. The other situation was, again behind glass, if we cut them in the afternoon and the next day or two were cloudy, sometimes they'd start to go moldy, and some of those turned black as they dried.
Works at a residential alternative high school in the Himalayas SECMOL.org . "Back home" is Cape Cod, E Coast USA.
The first time I had some blackened dried tomatoes, I thought they were a write-off. But even the most dark ones add an interesting flavor to sauces made from canned tomatoes. Kind of like having oven-roasted tomatoes without the energy use and fuss. Even in our solar dryer http://geopathfinder.com/Solar-Food-Drying.html that keeps direct sun off the foods, it is possible on really hot days to get a little burning, particularly if the tomatoes are high sugar content. If they are only slightly browned, you've just got some carmelized sugars to add depth of flavor.
How do they get the deer to cross at the signs? Or to read this tiny ad?