That is actually a good question, I have no Idea what it could be. We could go with the GameTheory that the Nether was frozen over a while ago and that is how Blackstone formed, similar to how Basalt formed when Ice and Soulsand got hit by Lava.
Aaaaand I just remembered you can infinitely generate Blackstone similar to how you can Basalt and Stone! So I cant just give something useful from that!
Any suggestions as to which things you should get out of Blackstone?