I prefer to use the blue pen to write though both of them are very good but blue pens are more used, while i was in secondary school I used both them because some times I love to be unique, so when majority are using the blue one, I opt for the black pen, in few cases I used both of them while writing note, I used blue pen for the most part of the note while the black pen for the keywords or definition.
Another important why I also use black pen is that sometimes when I type out documents, while proof reading I come in contact with some minor errors that a pen can correct, black pen is most appropriate since the color of the write up is black so it not be noticeable. I also prefer to sign vital documents using black pen so that I can be unique.