I didn't know about the size adjustments. Fantastic.
The paint mixer brush is interesting but I'm not sure exactly how I would use it. For blending I would be looking for something similar to what Painter or ArtRage has, which gives you a gradual transition from one color to another along the boundary. The Paint Mixer Brush seems like it would be useful to get an intermediate color that you could use between the other two but it still seems a little difficult to use for blending.
Many of the interviews I've seen with illustrators suggest that the common Photoshop way of doing it has been to use lower transparency once the main color areas are down, and then constantly resample color under the pen (with a shortcut key I would assume) and build up a smooth transition that way. That is a lot of work.
Thanks for the response and information.