The black apricorn is a feature introduced in Pokémon Diamond and Pearl, and its subsequent remakes, Brilliant Diamond and Shining Pearl. It is not present in the original Gold and Silver games, nor their remakes, HeartGold and SoulSilver.
It's possible you are thinking of a different type of apricorn, such as:
* Red Apricorn: Used to make Poke Balls that are slightly harder to catch Pokemon with.
* Yellow Apricorn: Used to make Poke Balls that are slightly easier to catch Pokemon with.
* Blue Apricorn: Used to make Poke Balls with a higher catch rate for Pokemon with a low catch rate.
* Green Apricorn: Used to make Poke Balls with a lower catch rate for Pokemon with a high catch rate.
* Purple Apricorn: Used to make Poke Balls with a higher critical hit rate.
These apricorns can be found in various locations in SoulSilver, though their availability is limited.