Gold of course, does not oxidise, so that is why it is used to plate electrical contacts such as these. If it looks like gold, it certainly should be gold, otherwise it not going to work too well. The black deposit would be from something else, perhaps from the underlying copper or a contaminant. As long as it is not thick, it should really be removed locally by the action of inserting the cartridge into the socket.
If you use an abrasive, the fine particles removed tend to scatter light rather than reflecting it, so look black. (Even finer particles are likely to be transparent and in fact finely beaten gold can itself be made to look transparent with a green hue.)
A game that is not working may indicate a faulty ROM, particularly if it is actually an EPROM. You could write an Arduino program to read the content and compare it to a known dump of the code - I believe there are a few of these on the 'net.