Why is it that blue pens are just barely secondary to the black, whereas greens, reds, and the like are hardly used except for corrective/decorative purposes?
I wasn't quite sure where to pose this question, but I figured here would be best, as I'm inquiring on the history of pens that led to this point.
The pervasiveness of blue ink has to do with the type of ink that preceded the modern dye based inks. From about the 5th century to the late 19th or early 20th century, standard ink used in Europe was iron gall ink. Iron gall ink, made from iron salts and tannic acid derived from oak galls, was superior to carbon based inks as it adhered better to parchment (i.e. didn't smear) and was waterproof. The ink was (and is) also valued for its permanence. The color of the ink is a dark blue/black, though it may turn to a brown over the centuries. The ink does have a drawback however. The tannins in the ink are corrosive and can eat away at parchment over long periods and more quickly with paper.
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.
If you want to discuss inks further, you should check out r/fountainpens or the Fountain Pen Network. You can generally find people on these sites who are quite knowledgeable about the histories of different types of pens and inks.
EDIT: Whoa, thanks for the gold!
The federal has government kept an ink database of all blue and black inks since the 1920s. This makes it possible to date a document for authenticity by the type of ink used. Additionally, until recently it was hard to produce colored inks that wouldn't fade. If you can find documents from the 70s or early 80s written in red ink they will have faded to brown. Many legal documents still require blue or black ink signatures only. All of those factors led the legal and banking sectors to prefer blue and black. If such professions find it acceptable, it trickles down to the common.