A buddy of mine keeps bluegills in his aquarium here in FL. He has been doing it for several months. Overtime, about 3-4 months, his bluegills seem to change color. They go from a very colorful fish, all the way to black. I can only assume it is because he is not caring for the fish properly. Last time I looked at his thermometer, it was on about 75F. I would agree with Azn Dreamer in that they need cooler water to live properly.
When they do turn colors, he throws them back into the pond and catches some new ones. He has tried to keep bass as well, but bass are an aggressive fish that needs to be fed live foods. Bluegills on the other hand will eat anything from lettuce to bread to crickets...