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

{
  "type": "minecraft:crafting_shaped",
  "pattern": [
    "###"
  ],
  "key": {
    "#": {
      "item": "minecraft:dark_prismarine"
    }
  },
  "result": {
    "item": "minecraft:dark_prismarine_slab",
    "count": 6
  }
}