Hi, You could in theory replace the white keys with black but you would have to get a silver vinyl for the wrist wrest and black vinyle bezel then you would have it looking like a unibody.
Well, if you were going to go really cheap how about Black Vinyl skins but wrapped in Carbon Fiber I did this with an old Compaq N800c. I went to Autozone and bought 2 roles of Carbon Fiber in black and patiently cleaned and applied the carbon fiber. You will need a hair dryer to encase the corners. Caution-the carbon fiber adhesive will leave a terrible residue if you try to remove it. Results made it look like a stealth laptop!