This one's pretty subtle, it will blend colors that are very similar while not blending edges at all.

I use it for cleaning up jpegs, or after changing to RGB mode from a 256 color photo,
or when gradients within the picture aren't smooth. Many times you wont see any difference
in the picture unless you zoom in, so don't assume that it isn't doing anything.

These are zoomed to show how ProBlend effects the picture:

< this to this >

Subtle, yes, but blended and still sharp.