minecraft-assets/assets/minecraft/models/block/polished_blackstone_brick_stairs_outer.json

{
  "parent": "minecraft:block/outer_stairs",
  "textures": {
    "bottom": "minecraft:block/polished_blackstone_bricks",
    "top": "minecraft:block/polished_blackstone_bricks",
    "side": "minecraft:block/polished_blackstone_bricks"
  }
}