The blotches come from biasing in the sensor, some of these patterns were caused by the manufacturing process, others come from the mounting inside the camera and others come from environmental factors. When you get near the noise floor of the sensor, these kinds of defects can become visible.

In order to fix this, take a whole bunch of flat files and dark files. I like to have a set I prepared one time, but also create a set each time I start a session. It doesn’t take long and helps with environmental noise. Then stack away at your data in order to boost the signal to noise ratio. These kinds of things should disappear if you do it right.