minecraft-assets/assets/minecraft/blockstates/polished_blackstone_slab.json

{
  "variants": {
    "type=bottom": {
      "model": "minecraft:block/polished_blackstone_slab"
    },
    "type=double": {
      "model": "minecraft:block/polished_blackstone"
    },
    "type=top": {
      "model": "minecraft:block/polished_blackstone_slab_top"
    }
  }
}