XD - with a stainless cylinder and barrel, and aluminum alloy frame, you are correct that bluing, which is a controlled oxidation process applied to the surface of carbon steel, would be difficult if not impossible to apply to the gun.
Parkerization is a baked-on process, IIRC, so if you really wanted to, you could apply it to the gun, but I can't see why you would. If you just want the gun black, black anodize the frame and apply a black teflon coating to the stainless parts.
Or buy a 438.
I just have to wonder - the 638 isn't going to rust, so what's the point?