I think the images in the article are mosaics created from multiple images and the dark blob in the center is where their was no imagery because Saturn's south pole is currently tilted somewhat away from us. It has a 30 year orbit, and you can see in the images we are able to see closer and closer to the south pole each of the three years images were taken. The rough edges are just an artifact of image processing, and where individual exposures cut off.

> Is it just me or does the dark blob in the center of the decagon look like a Mandelbrot set?

It's all just fractals all the way down?