I like this, but it just seems so blue.

Huh. I suppose you're right. It's become similar to Sakashima the Impostor plus Icy Manipulator almost. My original intent was to capture its method of knocking people out and stealing their identities for its own purposes, which it made feel black, but mechanics-wise it's seems blue. That's the trouble with top-down design, I guess.

That's pretty much the flavour of Dimir Doppelganger. And in fact the flavour of most of Magic's doppelgangers. "Stealing people's identities for its own purposes" sounds very suitable for blue.

And annoyingly, the other means of taking somethings identity - controlling it - is red.

Still, it's a legend, you can shift the colour pie a bit, maybe? Perhaps have it assassinate the creature it is going to become? (Which ups its powerlevel a lot)

I used to have another creature that killed its original, but I didn't keep it. I like the idea, but it isn't in flavor.