A personal answer I fabricated for that is: Poe wrote on both.

That's not the correct one, of course, but it's one I came up with and I thought I was pretty clever at the time because the difference lies in how 'on' is meant.

Another is that both croak, or that both are stained black, if you think about it and know the Raven's original color. But again, not the right answer. x3