Ooh, I have an idea. What if Honor created the Shardblades by trapping some of Odium's power, the way Preservation did with Ruin? Furthermore, let's assume that as long as the Oathpact holds, the bit of Odium in the blades is inert and can't corrupt the user. But at the Recreance, the bit of Odium was released to twist the wielder. So the Blades are of Odium, but Honor intended them to be "safe".