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!😄
Eurypterids give terrible amounts of black pearls and are very hard to find (On the island at least). Take a Basilosaurus (It doesn’t have to be great, mine was level 60 when I found it) and kill tusotuethis, Alpha Tusotuethis and Alpha mosasaurs. Alpha mosasaurs give 100 black pearls and normal tusos give around 30-50. Alpha Tusos give around 80. So, best thing is alpha mosasaurs, followed by alpha tusos, then normal tusos. OPTIONAL: Bring a dunkleosteus as a pack mule as they reduce weight of black pearls by 50%, so you can carry more and your basilo won’t be slowed down to a snails pace.