The only black diffusers ive seen are on ebay which match both the spoler and rear diffuser. They also have them in CF.
For CF - You can find those exact designs here:
Spoiler looks like 3D Designs ...
http://ind-distribution.com/ind/bran...ke=22&model=57
Rear Diffuser looks like Vorsteiner
http://www.vorsteiner.com/bmw/m-seri...rear-diffuser/