Working with layer masks and painting in an effect is a very powerful technique and can be applied in many different ways: white balance, saturation, levels/curves, sharpening, graduated neutral density, etc.... Spend some time working with painting in effects on separate layers. I generally use a moderately large brush with a very low transparency (I forget if that is the right name) of about 30%. The objective is to get a brush with a very soft edge that doesn't fully paint the black color until you go over the same space a few times.
Very powerfule technique. Play with it until you have it right, and it will be your friend for a lifetime.