Graham,
Tell me about it!!! Mine have gone the same and I wished I'd done something about it before they were fitted. Some discs come already painted but many don't these days. It seems the consensus is to paint the "hub" part of the disc with a zinc coating - it needs to be high temperature resistant and it's one of those jobs which is a real pain to do once the discs are fitted. I have very limited space and resources so it's just a pain getting the car jacked up, wheel off, wire brush the hub to get it clean and then mask the area and paint it. I really MUST do mine because, as you say, it looks terrible. If only I had garage space!!!
Regards
Dave