Some pics of a CB M sport with aluminum rails:
http://x3.xbimmers.com/forums/showth...522823&page=12
(scroll all the way down)
I think the aluminum rails provide great contrast to black paint as they highlight the kidney grilles and the wheels. I wouldn't worry about the rails not matching the window trim.
If you do decide to make a change, I would recommend you make the dealer correct it before you take delivery - just let them know that you won't sign anything until the car has black rails on it.