Are you selling it to him or buying it? I don't think a black cart variant was made for this (unless it's some UK version). If you ever see one my guess is someone did a case swap or colored it with a sharpie. I know some people use a sharpie to cover over any ugly game rental markings by just making the whole cart black.
Flea market guy must've seen one thinking it's rare or something.
I was trying to sell one and get some money to buy an snes, he told me that the gray cartridge isn't as rare and so its not worth much. the snes was only 60, I thought I could trade or at least get more than 35$
I buy and sell video games,and I can tell you there is no black Conkers Bad Fur Day cart,the game can be sold from 65 to 80 bucks on craigslist or eBay,if you have the box and instructions it can go for 100$ plus