GIMP :: Color Adjustment - Curving Certain Colors?

I draw images by hand and then scan them and digitally color them.

The white of the page is not a solid white, but full of variance.Also, there are many tiny specs of dust and things that show up as grey specs on the image.

Prepping a black-and-white line drawing is easy; I just use color curves to set the mostly-white areas to pure white, doing the same with black, and tweaking the curve how I want to make my black lines sharper or smoother and cutting out those grey specs.

But I've been trying to work out how to add a second level of detail in these drawings. This comes in the form of a light blue. This light blue can be used for so many great things, like points to line up an image that was drawn on multiple sheets or to represent lines that will becomes shadows or highlights that I don't want to show up in the final image.

Previously I had been scanning my pages as greyscale images to completely wipe out extra color variations that I didn't want. If I do that with the blue lines they turn grey and they get eroded when I adjust the color curves. If I scan it as full color I now have subtle color variants that throw things off, and adjusting the color curves still erodes those blue lines even worse.

If I try adjusting the color curves by color channel I turn the whole image to a particular tint.

How can I adjust the colors of an image to turn anything mostly white into white, mostly black into black, and mostly light blue into light blue; and preferably be able to adjust the scale it does this by so that I might prevent dust from becoming black spots and even maintain the greyscale range at the very edges of my lines?