To think of something that would be similarly out of place (but doesn't exist) think "Daedric Crossbow". See what I mean?
I guess. Oblivion is a vast place, though. I think that Daedric Katanas do and should exist, but that they certainly shouldn't be a part of any leveled/random item list.
If you look at the design of the Ebony Blade, it's a perfect example of a Daedric Katana.
EDIT: Also, wow I'm good at derailing these threads. Sorry.