I have a PBass that is the same way. As it is growing it is developing more and more dark patterning. I think it has more to do with the fish's mood, surroundings and it's place in the pecking order. Just like an angelfish can lose it's stripes, PBass can lose theirs.
Here's a thread about mine... http://www.monsterfishkeepers.com/forums/showthread.php?t=132296&highlight=kelberi
Here's a thread about mine... http://www.monsterfishkeepers.com/forums/showthread.php?t=132296&highlight=kelberi