On Java edition, veins of Blackstone in the various Nether biomes only generate below Y-level 28, meaning that patches of Blackstone are (almost) never exposed above the surface of the lava ocean (Y=30). As a result players must either dig deep down or find a Basalt Deltas biome in order to mine Blackstone.

On Bedrock edition, however, Blackstone generates below (approx.) Y-level 35 instead, and as such can be found quite regularly exposed to the surface.