The blackstone block and the blackstone wall are using the blackstone.png texture as the bottom texture, but the slab and stairs use the blackstone_top.png texture.
Blackstone uses side texture in the bottom
Sandstone and red sandstone walls have side texture on the top and bottom faces
Blackstone walls use blackstone side texture on the top face
Hoppers use the side texture on the bottom
Gilded Blackstone uses Old Texture
Top texture of gilded blackstone is the same as the side textures