There are several questions and a tag relating to what it is but I can't find much relating to how it is actually done in camera anywhere. The internet reveals many "how to" pages for doing the processing in software which seem to be broadly the same method ie.
-Take 1 or (ideally) more dark frames and use them as a subtraction or difference layer over the image or light frame .
This is mostly used by astronomers and is fine for dark areas (the space between stars) as the noise is reduced to black but obviously not so good for long exposures with lighter tones where black pixels will become evident.
To counter this two variations seem to be used
- Subtract the dark frame from the light frame then use a despeckle filter targeting the the dark pixels.
- Use the dark frame as a mask and apply some blurring to a duplicate light frame to interpolate values for these pixels.
Does the camera use something similar to the latter, perhaps using the dark frame as a mask then interpolating the data in a similar way to the way de-mosaicing works? Merely subtracting the dark frame doesn't seem to be what I see in the results I get as I see no black pixels on light areas of the image.