I hope they keep this as a feature and not as a bug, I collect tropical fish in my hardcore world, and the ones that are partly black do spawn naturally (1.19.3 and 1.19.4), and they are some of the coolest ones (attached image shows some of the ones i have found).
first observed in -3692434831046939593 (1.18). Occurs throughout the entire biome at aquifers, including nearby eroded badlands. Tested in 3 other worlds and the bug occurred in all 3.
Drigglesnork
I hope they keep this as a feature and not as a bug, I collect tropical fish in my hardcore world, and the ones that are partly black do spawn naturally (1.19.3 and 1.19.4), and they are some of the coolest ones (attached image shows some of the ones i have found).