I have factory black carpets as well. My headliner is light gray, and I have a blackish gray check-pattern for the cloth on my seats. My seats are the Recaro-style sports seats. Maybe that makes a difference with having black carpet?
Oh, and mine is an 85 325e.
You might be able to find carpet kits for your car from Bavarian Autosport,
www.bavauto.com. They're expensive to shop from, but they ship quickly.
Good luck!