So for example, if a black spell would cost , you may pay instead. Likewise, if a black spell would cost you , you may pay instead.

An easier alternative, but less cool IMO, would be to make it:

"Each black spell you cast has convoke."

I really want this card to be close to Necropotence levels of power. :[

Possibly simpler to understand:

­: Add to your mana pool. Use this mana only to cast black spells.

Spells you control have Swampvoke (Your Swamps can help cast your spells. Each Swamp you tap while casting spells pays for .)

Maybe not.

Why not have it "You may pay rather than pay the mana cost for black spells you cast."

Incredibly potent. But, well, it does cost five black - so maybe?

­Dream Halls would like to have a word with you. ;)

Color-shifted Dream Halls would be dope.