Artist Questions: Why Do Artists use Blue Pencil in Sketches?
A little while back, people would sketch with "non-photo blue" because it's more or less invisible when used on xerox machines. Now, since most people scan and adjust digitally, "blue-line" sketching is used by many artists out of habit and/or because it is an easy way to draw over the top of your sketch. The technique creates a background quality that allows you to build upon the drawing with pencil and pen without the bottom sketch getting in the way.
Example of "blue-line" sketching done by Comic artist, Scottie Young. See more of his amazing work here: http://skottieyoung.com/
The basic principle is to sketch in some color other than black. Once an artist is happy with a rough layout (done in blue or red), he/she will add cleaned-up lines in black on top of it. By adjusting color channels in Photoshop, the artist can easily get rid of the original sketchiness (done in blue or red) leaving them with only the cleaned-up black lines of the artwork.
Prismacolor Col-Erase Non-Photo Blue works great because it's easy to erase, but really any blue, red (or purple for that matter) will work!