minecraft-assets/data/minecraft/worldgen/configured_feature/blackstone_blobs.json

{
  "type": "minecraft:netherrack_replace_blobs",
  "config": {
    "radius": {
      "type": "minecraft:uniform",
      "value": {
        "max_inclusive": 7,
        "min_inclusive": 3
      }
    },
    "state": {
      "Name": "minecraft:blackstone"
    },
    "target": {
      "Name": "minecraft:netherrack"
    }
  }
}