Great question, element! And to all who answered, thanks: your advice deflected off of element and got me pretty good. I'm still trying to figure out how to make my stuff look more painterly, and trying to use hard brushes has resulted in a lot of garbage results. Unfortunately, it looks like there isn't yet an app or filter that replaces practice. Dang it.
Element, when you mentioned the 'getting darker with opaque brushstrokes' problem, it made me wonder if you had your brush (or layer blending property) set to 'multiply'. On a separate note (from someone who is still in the fetal stage of digital painting), this guy has some great tutorials, including a brand spanking new one about blending, right on the first page:
Thanks again to element and wise responders for a very valuable thread.
Will design for food, provided that we all agree that money is edible.