Current Tally:

Knifethrower – 1
LilyStorm – 1
ParadOxymoron – 1
Fakeartist – 1
Shazzeh – 1
Cato – 1

A six-way tie! TPzombieW, please cast the deciding vote.

Oh good lord. So much for consensus. The feeling of power this gives me however...

A good seal should be something you don't always want to crack immediately, and the presence of the seal should complicate the board state as minimally as possible.
Parad - Better than straight up removal seal. Easier to play around.
Cato - I like how this gets better with time/other seals. A little complicated for common and might put some limits on what the rest of the cycle should look like.
Lily - The effects match up and it's neat how this does something before going (creating a nice tension of whether you want to use it up or not). It kills way too many things for a common though.
Knifethrower - Good for responding to graveyard hate, even when tapped out, so it could find use outside the set.
Shazzeh - This is prety much the perfect ability to go on a seal. Unfortunately I don't think they do False Demise type cards at common. Being able to undo removal without even leaving mana open is a bit much at common.
Fakeartist - a nice rattlesnake card that warns the opponent not to attack with their best guy. Might slow down the game just a little too much though.

I vote for somebody else entirely!
I kid, Knifethrower takes this one. Choose your criteria, you crazy shiver, you.

It doesn't kill that many things ;-;

Unhallowed Pact was common, and that wasn't too long ago.

Make a black common instant that does NOT kill a creature. Something you would be happy to pick when drafting "spells".

Perish the Notion
Instant
Target opponent reveals his or her hand. You choose a non-creature, non-land card from it. That player discards that card.

[Yes, this is _almost_ a reprint]

Icy Whisper –
Instant (U)
Until end of turn, you may cast target instant card with converted mana cost 2 or less from your graveyard without paying its mana cost. If that card would be put into your graveyard this turn, exile it instead. Exile Icy Whisper.
Korinthia shivered. She had never been this deep into the caverns of Frostblight before. She couldn't see her hand in front of her face, but she could feel her breath freezing. And she could swear she was not alone.

@Parad- That doesn't seem very black.

I just figured, Sins of the Past.

Vampiric Infusion
Instant
Target creature gets +1/+0 until end of turn. Until end of turn, whenever a creature dies, put a +1/+1 counter on that creature.

Mindshivers
Instant (C)
Reveal the top five cards of your library. For each of those cards, you may pay 1 life and put that card in your graveyard. (Put the rest of the cards back in the same order.)

Harsh Reprimand
Instant (C)
Counter target non-creature spell unless its controller pays 2 life.

(Edit: Renamed it from Excruciating Feedback. Harsh Reprimand sounds more like a black card.)

@Shazzeh - Doesn't seem common.

Something infusion
Instant.
Regenerate target creature.
Target player puts the top five cards of his or her library into his or her graveyard,you gain 1 life for each creature card put into the graveyard this way.
"A big enough sacrifice and the injuried will be saved, anything extra will be our reward."Inscription on the temple offer plate

Reprint Cremate, I suppose.
Everything I can come up with is either unplayable or uncommon.

Leech Vitality
Instant
Regenerate target creature. Each opponent loses life equal to that creature's toughness.

Reprint Songs of the Damned

Songs of the Damned
Instant (C)
Add to your mana pool for each creature card in your graveyard.
Not wind, but the breath of the dead.

This is outside Black's current color pie.

Frostwynd Avalanche --
Instant (C)

Target player puts the top X cards from his or her library into his or her graveyard.

I dunno, doesn't it? It's like a weird cross of Orcish Spy, Index, and Mulch, and all those cards are common. I don't think it's doing anything that complicated or unsafe.