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.
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!😄
Ragnarok in Viking Bay around the sunken ship in the middle of the bay and also at 10 90. There’s a tiny tip of a Boulder to land a flyer out there. The water is shallower in that area. Another sunken ship there with lots of lobsters to kill.
Aberration down in the red hazard zone. They can be picked up off the ground down there.
Genesis in the volcano biome. Down in the Magmasaur pit they can be picked up off the ground there too.