its been my experience, as was stated, red belly pacu seem to turn black when being stressed. be it another tank mate, sickness, or water quality.
the color range of their silver/gray and their reds ive seen change here or there for no apparent reason. but going pitch black, if it is a red belly, is a sign of stress.
one idea would be get some real large terra cotta flower pots if you can fit them in your tank, and try to give the pacu a chance to hide a bit. if he's getting picked on that badly, your going to have to try and find another tank or home for it. eventually it will most likely lead to the fish's demise being stressed out 24/7