If you ask me I would say that it contrasts better with the documents you're writing on.
Now coming to the technicalities -
Originally paper was khaki color but Chinese in particular managed its color. Blue color has been easy to produce for more than thousand years (as compared to any other color), later blue color also became common and people started having a sense of better contrast as compared to black.
When paper copiers appeared, blue writing has gained even more importance. Original and copies are easy to distinguish in case same paper has been used in both cases.
