I remember reading something about a 'black blade' class feature for the swordmage but now I can't find it. It was basically where the 'black blade' was an intelligent weapon with it's own agenda. I was thinking of using it for the game tonight, but I can't find the information on it, and the GM won't let me play it without a source, not that I blame him, not much to go on with what I remember.
Are you sure you aren't thinking of the Black Blade from the Blade Bound Archetype for the Magus in Pathfinder? It's detailed as exactly what you put here.