system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are most pens blue or black ink? Shouldn't they just all be black, or was there a reason that blue became popular?
gpt
From about the 5th century to the late 19th or early 20th century, standard ink used in Europe was iron gall ink, which is dark blue. 

In the 20th century, ink manufacturers started to develop other formulas for inks, synthesizing new dyes that would not corrode the metal parts of fountain, dip, and ballpoint pens. New inks, both waterproof and non-waterproof, were introduced and in an array of synthetic colors. Iron gall ink continued to be used for much of the century, though, particularly on governmental documents, due to its superior permanence.

As stated, the color of the ink ranges from a dark blue to a purplish black and modern usage of predominantly blue and black inks is in imitation of the older ink. Blue ink is considered acceptable in situations where other colors would not be because of the longstanding cultural precedent set by the blue tone of iron gall ink.