Fact: Blue Ink
does not photocopy
This is a basic principal behind special pencils and pencil leads that allow draftsmen to create reference lines (also called construction lines). These lines don't reproduce at all. It saves the draftsman a lot of time and frustration because the lines don't need to be erased. And the final product looks clean and well drafted.
For Diaso processing (Blue Print Machines), one can use Non-Print Purple in the same manner.