This kind of thing fascinates me:
First, Marc Tyler Nobleman -- whose book on Jerry Siegel and Joe Shuster, Boys of Steel, has just been published -- notes a significant, but totally expected and unsurprising, omission from the credits of a recent superhero movie, leading him to wonder about the origin of the phrase "the Dark Knight" before it appeared in Detective Comics #40 in a story written by Bill Finger in 1940.
Then, doing the literary legwork, J.L. Bell traces the coinage back to Irish gothic novelist Charles Robert Maturin in 1816.
What, you didn't think Frank Miller invented it, did you?
Found here. Both blogs are really excellent in general and, it should go without saying, well worth reading.