hmmmm, I can't think of any reason why they would have black splotches or why they are increasing. It is really had to diagnose fish without looking at them.
However, I will say that I have experienced Rift Lake cichlids with some black type fungus growing on them, it is very suttle, when you look closely you can see how it goes around the scales. It doesn't seem to ever hurt the fish, as they contineud to grow, and even spawn with out an adverse effects. So I would suggest looking at it hard and perhaps even taking a fish out and putting it under the microscope if you have one. You may even want to try to see if you can scrape it off.
I saw some pictures of frontosa once that had what appeared to be black splotches, and it was actually black scales. The hobbyist removed the fish to take a closer look and it was realized that they were just dead scales that for some reason didn't fall off, and once scraped, new scale grew back with out any black. It was determined that this was an ammonia burn from when they were shipped.
Also I will mention that I have heard of fish getting black splotching from too much spirulina algae, I have never experienced this, and wonder how that could even be, but since this is probably what you are feeding your fish, it would be something to consider. It may be a matter of just chaning brands of food.
As long are your fish are acting normally and eating good, I wouldn't worry about it too much about it, and if you are the scientific hands on kind of person, take a closer look.