I have tons of them that color...could be from breeding color, displaying a fake dominance color so others wont mess with him...also could be from being stunted...tons of diff reasons why... but its normal....
My JD looks exactly like yours. It is in a tank with two Oscars and one other JD. The black one is the dominant of the two JD's and the Oscars act like neither JD is even in the tank with them.
I don't see anything wrong with it. My non-dominant JD is a pale coloration.