>I've read (but haven't had a need to try it so far) that grain in sky areas
>can be corrected/retouched using the blue chanel as a mask. I'd be
>interested in hearing from those who've used that technique.
I have read about performing some filter operations just on the blue channel
but have not tried any of those techniques.
One technique that I do use I first read in a 1996 Adobe (Photoshop) article
titled Repairing Digital Photographs by Russell Brown. (It used to be
available in a pdf on Adobe's site but I don't think it is there now.)
Duplicate the image on a second layer. Apply Guassian blur to this duplicate
layer. Brown writes "Choose values based on the resolution of the image. You
want to soften the image—not lose the image completely, but soften it —
especially in the troubled area of the blue channel." Then change the blending
mode of this layer to Color and flatten image.
You can test the effect of this procedure by looking at the blue channel before
and after. I'll see a lot of noise there "before." With a good selection for
blur amount that noise will be substantially reduced after the above procedure.