If you killed the Necromancers inside the cave of Dark Fissure, you should have looted a note that says this.
"To the Achorites:
Place your Grand Soul Gems in the altar. Pray for His Blessing, and strike the altar with your Soul Trap magic. Your gems will become instruments of His Divine Power."
But I've never tried the Black Soul Gem thing myself.
"To those who claim they will die for their country...
What the %#($ is wrong with you!?
Make the enemy die for theirs!"