Dark color in goldfish almost never stay ... My black moor is now almost totally gold/red. (And he is only 9 months old .... moors usu. keep their black color for years they gradually turn brown.)
Black is generally a juvenile color, & the gold will show through as it grows. In show fish, blacks in an adult is prized (& will fetch a high price).
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.