Use whatever you can afford, and helps you make photos. What comes out of the camera is far, far more important than what's on the surface of the camera.
And if using a chrome back on a black body bothers you bad enough, it's not too hard to peel the leatherette off carefully, and paint it. Because I'm fairly sure it's just the same chrome as on chrome bodies under the black paint.
You *could* apply a durable black finish to your back, but I don't think you need to be too particular--Hasselblad paint jobs aren't terrifically hard-wearing themselves. Personally, if I had to do it all over--and didn't get such a good deal on my 500CM kit--I'd have gone for chrome, partially because I'd never have to worry about the paint.
I've got a black 500CM and a chrome 553ELX and 500ELX, and really, I kinda prefer the look of the chrome over black, anyway.