1. Use adjustment layers to begin cleaning up your line art.
    • Remove any non-repro blue underdrawings with a Hue/Saturation adjustment layer: choose the Cyans colour range in the Properties panel and set the saturation to minimum and lightness to maximum.
    • Use Curves and/or Levels adjustment layers to remove any remaining underdrawings, and to improve the line quality and whites.
    • For high-DPI inked line art, use a Threshold adjustment layer to convert the line art to black-and-white bitmap. This makes flatting easier.
  2. At the bottom of the Channels palette, click on Load channel as selection to create a selection of the white areas of the image.
  3. Invert the selection.
  4. On a new layer, use the Paint Bucket tool to fill the selection with black.
  5. Deselect the selection, and hide the scanned artwork and adjustment layers. (I prefer to hide these layers instead of deleting them so I can refer back to the adjustment values I used.)