Blue pencil (editing)
A blue pencil, also known as a checking pencil, is a two-color pencil traditionally used by an editor to correct a written copy. The blue end is typically Prussian blue, and the red end is typically a warm vermilion red. They are most often half red and half blue, but some are 70% red and 30% blue. An editor-in-chief would use a blue colored pencil to make proofreading marks and final notes on manuscripts before sending it to be typeset and published. The pencils and their blue excisions became associated with the editing process and editorial oversight.
Of course it takes skill to be a poet! But an editor? A pair of shears, a blue pencil, and a paste-pot!
Since the introduction of desktop publishing, editing is typically done on computer files and without literal blue pencils. They continue to be used in the Japanese newspaper industry and elementary schools in parts of Europe. A different type of non-photo blue pencil is used by some comics artists for different purposes.