Blue ink has traditionally been preferred (though this is not a legal necessity). This is because blue ink quickly distinguishes between an original and a copy. Black ink might be mistaken for a photocopy, especially when the ink fades over time. The quality of today's blue ink is very good, so the difference between an original document and a copy is clear even after some time has passed. However, since black ink is also available in fine-quality cartridges, it is possible to create high-quality copies with black ink as well. In addition to being easier to see through, blue ink comes out of most pens, which means that paper clips, stamps, and other writing utensils are required to use red, yellow, or green ink. Some people think that using blue pen is unprofessional, but this is not true. As long as you keep this fact in mind, using blue pen will not affect your reputation at all. In fact, many professionals use blue pen because it makes their work look more official. There are some professions where using blue pen would be inappropriate. For example, if you work for a police department and use blue pen to sign off on arrests, this could be considered evidence of wrongdoing. The same thing goes for doctors' offices and other places where signs-off are needed to confirm actions taken during visits.