Well if you are going to use coal crystals, which is not really relieable...
There is stuff like opal blue, and moon stone that is very dark but not exactly black, silver and actually gold crystals/diamonds... not sure that is posible unless you place a silver background or gold background behind the clear diamond.
if it had gold fragments in teh diamond/crystal would result in the thing being rather unstable and would most likely be having an easier way of breaking into pieces or exploding during harsh conditions of leading light or power through them.
A jedi or sith or whatever force faction we speak of would under no circumstances weaken his weapon by taking a risk in doing this and that, just my five cents.. you can compare it to the way the Katanas and in general samurai swords was made in the feudal age of Japan, prime materials, and several layers of steel, the technology by that time is a mysterie, but also a great development and very insanely skilled.
Black/silver or gold sabers is just a bling bling created by the new age force users imagination and added to a game, though it never should had existed in the ways it is now.
Just paying my respect to the force user idealogy, not otherwise.