Here's why it's not incredibly rare:
* Mark VII LSC Production: The Mark VII LSC was produced for several years, resulting in a substantial overall production run. While specific color combinations were limited, the total numbers were still relatively high.
* Popularity of Black: Black was a popular color choice for luxury cars, meaning a fair number of Mark VII LSCs were produced in black. Adding black leather and black rims increases the rarity somewhat but not dramatically.
* Availability of Aftermarket Parts: Aftermarket black rims could have been added later, making it hard to determine if the black rims were original.
To find out how rare *your specific* 1991 Lincoln Mark VII LSC is:
You'd need to find a resource like the Lincoln Owners Club or dedicated Mark VII enthusiast forums. Members there might be able to help you based on the vehicle identification number (VIN). Even then, exact production figures for specific color/option combinations are usually not available to the public.
In short, while a black-on-black 1991 Lincoln Mark VII LSC with black rims is *less common* than other color options, it's not considered a particularly rare collectible car in the collector car market.