Blueline is used in various kinds of graphics meant for duplication, when you do your pencils/drafts in blue or cyan and then ink over them in black. (Most art stores sell blue pencils just for this, though these days someone doing that is more likely to scan black pencils, computer manipulate them to blue, and then print a blueline copy to ink on.)

This is traditional because some kinds of early duplication processes made it easy to filter out the blueline and only get the black inks, kind of like greenscreen in film - if you've ever seen original art of old comics, for example, you can usually see the blue pencil lines under the ink, which wouldn't get picked up when they were converted for printing.

In Photoshop there's a method that works pretty well to filter out the blue pencils if you scan in an inked graphic; but there's something about GIMP's equivalents of those filters that just never worked nearly as well, no matter how much I fine-tuned them - it would either leave in too much of the blueline pencils or erase too many of the bluish pixels in the inks, and I never found a good tutorial on it for GIMP. But it's possible that there's a GIMP tool that does it better than PS that I just never found.