Okay according to some of the old posts on AA the darkening is most likely melanism. Melanism in other animals is a genetic change causing animals to darken as they age and their offspring have a good chance of carrying on that genetic trait. (ex. Black squirrels) According to the sites about fish it is especially common in clowns. Carey's black clowns are an example of melanistic clowns that are genetic. Some people suggest that this is happening in cases like yours as a chemical reaction to certain corals and/or hosted anemones. Food was sited as a possible trigger. Others say it's just part of the genetic aging of particular fish. All of the sites suggest that it is not harmful for the fish and that it shouldn't trigger changes in behavior. The change in behavior, IMO, is just the growth and aging of the fish. It is now big enough to be aggressive and I think that sounds like more of a potential problem than the color change.
