Black blotches can be caused by a parasitic worm. Its easiest to see on the rim of mouth and the leading edge of fins. The 12 gallon nano is good for a pair of Oce's; that's not causing back blotches. If its outside normal color patterns for Percs and they are truly Oce's, I would be concerned. Make sure the water is right. Greenstar growth is not an indicator of water quality. If the tank has been set up for a while, it can approach toxic levels of some elements unless there are water changes. If you have softies, they can mess the water up with their own chemical toxins trying to compete for their space. Everything has its chemical defenses. If water is pristine, I would really start asking about a problem. Colors may darken or fade, but blotches only show up in response to something. Oce's don't blotch on light changes either like some fish do.