minecraft-assets/data/minecraft/recipes/dark_prismarine.json

{
  "type": "minecraft:crafting_shaped",
  "pattern": [
    "SSS",
    "SIS",
    "SSS"
  ],
  "key": {
    "S": {
      "item": "minecraft:prismarine_shard"
    },
    "I": {
      "item": "minecraft:ink_sac"
    }
  },
  "result": {
    "item": "minecraft:dark_prismarine"
  }
}