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

{
  "type": "minecraft:crafting_shaped",
  "category": "building",
  "key": {
    "#": {
      "item": "minecraft:polished_blackstone"
    }
  },
  "pattern": [
    "###"
  ],
  "result": {
    "count": 6,
    "item": "minecraft:polished_blackstone_slab"
  }
}