Here's probably a very simple question for a lot of you, but has left me
curious because someone in my office asked me this question (some of his
excel filename's appeared blue), and I didn't have a clue.

Please somebody explain to me why a filename (any MS Office filetype) is
sometimes displayed in a blue format instead of the usual black?

How useful is this characteristic other than making the file stand out
easily in a long list of similar files that have the black filename format?

Answers appreciated, thanks!