Recently I noticed some hip hop artist wearing blackish looking
chains and charms
Neither gold, nor any of it’s alloys, is black in color. That
includes those chains and items you saw.
Black colored metal, with gold or something else, is generally a
surface patina or a coating. with silver, it’s easy and common to
blacken the surfaces with liver of sulphur or another oxidizing
solution. With low karat golds, you can do similar things with
commercially available gold/silver oxidizers, which are a bit more
than just liver of sulphur.
You can also use electroplated coatings, such as black rhodium,
black nickle, or similar electroplated coatings. Some of these are
better than others in terms of durability, but it’s an issue with
most of them.
In recent years, we’ve seen high tech coatings used to get various
colors, black, brown, purple, red, and others, on any number of
jewelry metals. Some of these are vapor deposition coatings, but the
majority of those chains that I’ve seen, seem to be a sort of ceramic
coating. Not sure exactly what. But this isn’t simply a “studio
goldsmith” sort or technology, and it’s not dependent on the alloy