What is a non-photo blue pencil?
It has to be a particular shade of blue.

From Wikipedia, it is apparent that non-photo blue refers to a particular colour rather than a particular kind of pencil:


  Non-photo blue is a particular shade of blue that cannot be detected by graphic arts camera film. This allows layout editors to write notes to the printer on the print flat (the image that is to be photographed and sent to print) which will not show in the final form. It also allows artists to lay down sketch lines without the need to erase after inking.


Here's the code for this colour under various systems (also taken from Wikipedia):


Hex triplet:    #A4DDED
sRGB:   (164, 221, 237)
HSV:    (193°, 67%, 79%)


And here's the colour itself: