The plant below is 'Moon Child' and is red backed and so the birthmarking shows up as darker than the usual coloring of the plant.
So, how does this birthmarking occur? It seems it is a genetic defect. It was explained comparing this happening to humans having heart disease in their family. And it doesn't always happen to all plants or humans in the family. My Mom and her brother had heart valve replacement but not either of their sisters. So it happens to 'Super Duper' but not all the time and not to all of the plants. But, it happens more often to this cultivar than to other African violet cultivars. Taking a birthmarked leaf and propagating it will make a birthmarked plant. Using a plain leaf from the same plant might result in a birthmarked plant and might not. Does any of this make sense? I hope so. I'm trying to explain it so it is easy to understand.
I think these plants are amazing and more beautiful than the plain plants. But, as stated before, I love variegated plants. So, if you see a plant like this that isn't supposed to be variegated, don't throw it away thinking it is diseased. It is just different.