Chris, I've found that moving the tone curve around has and equal, if not better effect than just the black slider. By moving the Highlights to +100, the Shadows and Darks to -100 and then playing with the Lights slider between +40 thru +80, I seem to be able to get a better range to see the dust spots.
And then when you've finished, just reset all the sliders to 0 - nothing to remember.
