Just go to eldrich zone on crystal isles and look for big bubbles that have some black clams/oysters inside of them, put your angler fish inside the bubble, harvest, and enjoy the best black pearls harvester in the entire game, super easy, rank up for people to see this.
You can find these easily in the space biome on Genesis 2. Wait until the biome has dark grey/black asteroids with big light green/pink balls floating around (ignore the colour bit cos I kinda forgot what colour they are, but you get the idea) . You should find some little balls on the ground, which you pick up. They will give mostly black pearls and sometimes mutagel.
If you’re on ragnarok, go to the sunken ship with a spino that has a couple points into weight and damage , then farm the eurypterids that spawn there for insane black pearls. Since they spawn so often, you can keep going there are get way more black pearls that you’ll need
When you want black pearls in great quantity, here are some things I suggest.
1: Ragnarok: Vikings bay. Bring a good crossbow to one shot any water scorpions, they drop TONS of black pearls.
2:Extiction: Gachas. Sometimes when taming gachas, you can get one that produces black pearls. They produce in big quantities, and for me is by far the safest and easiest way to get the black pearls. Like if this helps!😄