Minecraft wiki entry for Advancement_guide, associated text: If you are in the Nether or below Y = 16 you can also mine blackstone or cobbled deepslate to get this advancement although the chances of you getting to the Nether or finding a deepslate blob without picking up any cobblestone are very slim. Blackstones are located in the basalt deltas biome. You may also find blackstone in other biomes near the lava ocean.