There are a few places where you can find black pearl shells on the Ragnarok map of Ark Survival Evolved. A few of the black pearl shells are just laying on the ocean floor, but most are found in old shipwrecks on the western side of the map. There is a rumor of another location, but when I went there, I found no black pearls.
Ragnarok Black Pearl Locations
Another great way, and perhaps even better way to get black pearls on Ragnarok is fighting Eurypterids in Viking Bay with a frog. It is probably one of the best places in Ark. See more in this video.
21.9 33.4 Frog Raft