Blackstone is closer to Obsidian considering how shiny the rock is. And Netherlicious has a special Spectral Dew Cobblegen alike Recipe for Blackstone, which by the way also has an Extruder Generator in GT6 that DOESN’T require Spectral Dew to exist, so it also works for Et Futurum Requiem. But yeah its available through bartering too in Vanilla, so the Cobblegen way is not that exploity.
Not to mention Soulsand is already the Coal thing, so no doubling up on that one.