minecraft-src/data/minecraft/worldgen/configured_feature/basalt_blobs.json

{
  "type": "minecraft:netherrack_replace_blobs",
  "config": {
    "radius": {
      "type": "minecraft:uniform",
      "max_inclusive": 7,
      "min_inclusive": 3
    },
    "state": {
      "Name": "minecraft:basalt",
      "Properties": {
        "axis": "y"
      }
    },
    "target": {
      "Name": "minecraft:netherrack"
    }
  }
}