Here is some data on black enchantments issued to date. I came up with 167 from Gatherer. Here is the mana curve.
This is the casting cost.
...and the major theme. It looks like Graveyard, Like, and Sacrifice are the big three themes.
From these, I kept for study the ones with the smaller text.
A lot of design ground has already been covered. Here is one of my favorite black enchantments.
And here is my submission:
Pay 3 life.
Sacrifice a creature with converted mana cost 3 or more.
Return a creature card from your graveyard to the battlefield.