Blobfish only care about water below level 35, biome settings do not affect it. This causes them to spawn in pools of water in caves, or even in the end if there is a spot of water for it.
They can also spawn a lot if they are the only fish for the area, as shown in the screenshot below. A to bedrock pond in a dark forest biome, and soulsand bubbles are pushing them to the top

delete the config file blobfish_spawns.json