Using GIMP's color channels to remove blue guide lines from inked line art

Penciled comic book art typically have blue guide lines (and text) as shown in the screen shot below. 

The sketch can be downloaded from this site https://www.deviantart.com/edtadeo/art/Elektra-2-Pencil-174307445 if you want to practice with it. There are a number of ways to remove the blue lines from the image. I will be using the Color channels panel to remove the guide lines.

The steps:

Remove pixels from the Red and Green channels

1. In GIMP, open up the image. Click the Channels tab on the bottom right.
   
   
   
   
2. Click the Blue channel to deselect it.
   
   
   
   
   
3. In the Tool palette, click the Fill icon.
   
   
   
   
4. Toggle on the FG color fill. Make sure the foreground color is black since we want to remove the Red and Green channels data.
   
   
   
   
   
5. Then, click anywhere on the canvas.
   
   The background becomes blue.
   
   
   
   

Create the line art in the Red and Green channels

1.  In the Channels tab, press the mouse right click button on the Blue channel.
   
   A popup menu appears.
   
   
   
   
    
2. Choose Channel to Selection.
   
   The blue channel's non-zero pixels are selected.
   
   
   
3. In the Tool box, click the Fill icon. Toggle on the BG color fill option. (Note: the background color should be white). Then click anywhere in the selection on the canvas.
   
   The background becomes white.
   
   
   
4. In the Channels tab, click the Blue channel to select it again.  Then in the menu, choose Select | None to clear the selection.
   
   
   
   
   
   

Use Threshold to clean up the line work

1. In the menu, select Colors | Threshold.
   
   The Threshold dialog box appears.
   
    
2.  Drag the black triangle until you are satisfied with the contrast between the line work and the background.
   
   
   
3. Save your work.