Disenchants:
[[Caustic Caterpillar]] [[Viridian Zealot]] [[Reclamation Sage]]
In these cards we have two different manners of getting rid of artifacts and enchantments. Reclamation Sage is easily tutored and used on the spot in response to difficult permanents, which Viridian Zealot and Caustic Caterpillar can be placed as deterrents. Additionally, they can also get around effects like [[Torpor Orb]], which can haze a graveyard focused creature deck, as a lot of our cards tend to have ETB effects. Reclamation sage can't deal with the orb, but our other two cards can.
Graveyards Matter: [[Satyr Wayfinder]] [[Stinkweed Imp]] [[Splinterfright]] [[Nighthowler]]
Satyr Wayfinder is sometimes worse than an elf, and sometimes many times better. It doesn't ramp you, but it can do a few things. 1: It can dig you to the color of mana you might be missing or need. A three color deck that runs a lot of basics can be very shaky on its manabase. In addition to ramp creatures, having access to a cantrip that gets us a land is very useful in shoring up the first few turns of a game. 2: It can get the graveyard train rolling. Sometimes, and often times, depending on the build of your deck, you can start milling yourself and going off with dredge cards. Satyr Wayfinder is a great way of looking for that early spike.
Stinkweed is one of a few dredgers that most serious Black Green decks really need. For the cost of 5 cards off the top of our library, everytime we could draw a card, instead we could put the imp back into our hand. Like [[Golgari Thug]], we can sacrifice or chump block with the imp and keep digging deeper and deeper into our libraries. It's a great deterrent, and a great shovel.
Splinterfright and Nighthowler are innocuous enough at first glance. They can get pretty big sure, but how big can they get? Depending on your build, you could be sitting on 20 or more creatures in your graveyard by the midgame. These two, are MASSIVE. Each also has their own special quirk about them. Splinterfright gives you an additional mill trigger like [[Nyx Weaver]] and can just trample over on its own to destroy someone. Nighthowler can be put onto an evasive creature for even easier damage, and more survivability since they'd have to kill it twice. Nighthowler also has the added benefit of making your commander a one hit kill. You didn't plan for Voltron, but you've got it, and sometimes you can just straight up kill someone.
Slaughter the Weak: [[Merciless Executioner]] [[Fleshbag Marauder]] [[Bone Shredder]] [[Shriekmaw]] Shush, it counts [[Stronghold Assassin]]
Executioner and Marauder both work as edict effects on a stick. You can sacrifice themselves to the trigger, or other cards you deem less useful on the field, or more useful in your graveyard. Some examples of this are dredge cards, or cards with enter the graveyard effects.
Bone Shredder enters and leaves the battlefield as a [[doom blade]]. It's not much of a help against black creatures, but it has a wide range of targets. We don't have to worry about removing it for reuse later, as the echo cost takes care of it for us.
Shriekmaw isn't a 5 drop. It's a sorcery speed [[doom blade]]. See Boneshredder. Additionally, Shriekmaw when played for its full cost or cheated out, is a great target to bestow a [[Nighthowler]].
Stronghold Assassin gets rid of our weak, useless mana rampers and things we'd rather have in the grave, and turns them into sweet, sweet, [[Doomblade]]s. He's also another great way to get our dredgers into the graveyard. He can even keep using them as ammunition.
Card Draw/Tutors: [[Heartwood Storyteller]] [[Fierce Empath]]
Storyteller is great in a deck that's mostly focused around creatures, but can be a significant drawback if your deck plays with more noncreature spells. My particular list only runs 8 noncreature spells. The storyteller is also a great political tool, letting other players draw cards. If a player has more spells than creatures in their decks, they're going to start eyeing you and your little bard with disdain, however you can use this to your advantage. Your goodwill and free cards are leverage to use on other players to work with you, and against the nasty combo decks that threaten you.
Empath is kind of a cantrip in a way. It's easy to get our big answers and threats in a graveyard deck, but sometimes you just need the extra help. Empath is a short stop away from big players such as [[Avenger of Zendikar]], [[Craterhoof Behemoth]], [[Grave Titan]] and many other big threats. If you're running [[Rune-Scarred Demon]], Empath is only two steps away from any card in your deck.