Since your deck is quite dependent on having your commander in play, as Hunding Gjornersen mentioned, you probably want more cards dedicated to keeping it in play.
I'd encourage you to include some of the following to protect your commander:
Apostle's Blessing
Benevolent Bodyguard
Blessed Breath
Cho-Manno's Blessing
Coalition Honor Guard
Crimson Acolyte
Hyena Umbra
Moonlit Strider
Obsidian Acolyte
Pentarch Ward
Shelter
Standard Bearer
Stave Off
Ward of Lights
You may be leaning too heavily on the ramp aspect of this deck. 40 land plus I count an additional 26 additional cards that can fetch lands/ramp mana. Seems that this deck runs the risk of flooding out more than I'd be comfortable with. If you are squeezed for cards as you've mentioned earlier, I'd look at clearing out some mana fixing/ramp.
In addition to Bloodfire Infusion, for board sweepers you can make use of Pestilence and Crypt Rats (and to a lesser extent, Gangrenous Zombies and Swirling Sandstorm).
For mana fixing, Terramorphic Expanse and Evolving Wilds can help out as well.
Given the number of ETB triggers you've got (especially your commander), you might consider Angelic Renewal and Whitemane Lion in addition to your Cloudshift/Otherworldly Journey package.
Shade's Form is either a nice upgrade to Unhallowed Pact or another copy you can add to the deck. Added utility where you can turn one of your own creatures into a threat.
Very cool deck concept. Mono-black control seems like a fine shell to stick this in.
Some quick ideas to help flesh out the deck, control elements to keep you alive and card draw to get you to your win condition.
Mono-black control elements:
Pestilence
Crypt Rats
Dimir House Guard
Evincar's Justice
Gangrenous Zombies
Corrupt
Pristine Talisman - Ramp you while ensuring you have a higher life total.
Wall of Shadows - infinite blocks
Hymn to Tourach
Faceless Butcher
Augur of Skulls
Chittering Rats
Gravedigger
Tendrils of Corruption
Ichor Slick
Geth's Verdict
Hideous End
Card draw/Deck manipulation:
Darksteel Pendant
Skulltap
Altar's Reap
Sign in Blood
Phyrexian Rager
Moriok Replica
Arcane Spyglass
Lose Hope
Fill with Fright
Casting of Bones
Long list of possible cantrips:
Afflict
Instill Infection
Expunge
Mind Stone
Gempalm Polluter
Chromatic Star
Cremate
Conjurer's Bauble
Dirge of Dread
Aphotic Wisps
Necrogen Spellbomb
Nighthaze
Gravebind
Headstone
Ichor Wellspring
Chromatic Sphere
Elsewhere Flask
Prophetic Prism
Recover
Relic of Progenitus
Sicken
Syphon Mind
Unburden
Unearth
Disciple of Malice
Hunding Gjornersen is correct, both Shieldmate and Stealthsuit are fantastic general protection. I somehow overlooked them, despite running both in Gilder Bairn and Mistmeadow Witch.
EDIT: Just thought this through, Aboshan's Desire gets pretty bad and stops protecting when you try to go "all in" and remove your graveyard to feed the 'Tog.
Convolute is fairly weak. I'd rather run the cheaper Mana Leak over it. In my blue Pauper EDH decks, I've been a big fan of Arcane Denial. It's a hard counter for 2 mana and its draw ability even plays well in your deck.
Stinkweed Imp could fit in well. Early chump blocker and its dredge can really fill up the graveyard, especially with all those looters.
Why are you running the artifact lands and Expedition Map? I don't see any interactions with them in your deck. Are you meaning to run Trinket Mage? That would work well.
The inclusion of Aboshan's Desire along with the obligatory Whispersilk Cloak is nice. Obviously you'll want some piece of mind knowing you won't lose everything when going all in with Psychatog. While it might not make the cut, the next best in this category is likely Mizzium Skin.
AEthersnipe is a Pauper EDH champ, leaving a beefy 4/4 body behind after bouncing a permanent. Recoil is likewise a well-rounded utility removal spell. Even if the deck weren't so blue, Drain Life and Consume Spirit are clunky, expensive removal spells that could probably stand to go. Snuff Out is a good contender.
Mind Stone is cheap ramp that also provides late game upside if you are flooding out.
Excellent deck, very well thought through. If you are looking for more removal, I would definitely promote the Arachnus Web as the next most appealing option. For so many utility creatures and generals, taking away their activated abilities is clutch.
EDIT: Presence of Gond is another great addition. Works alright as an untap target for Symbiote. Does a fair job boosting your cards that count elves. Also, it allows for infinite creatures if you stick it on Devoted Druid and have an Ivy Lane Denizen in play.
I'd encourage you to include some of the following to protect your commander:
Apostle's Blessing
Benevolent Bodyguard
Blessed Breath
Cho-Manno's Blessing
Coalition Honor Guard
Crimson Acolyte
Hyena Umbra
Moonlit Strider
Obsidian Acolyte
Pentarch Ward
Shelter
Standard Bearer
Stave Off
Ward of Lights
For mana fixing, Terramorphic Expanse and Evolving Wilds can help out as well.
Some quick ideas to help flesh out the deck, control elements to keep you alive and card draw to get you to your win condition.
Mono-black control elements:
Pestilence
Crypt Rats
Dimir House Guard
Evincar's Justice
Gangrenous Zombies
Corrupt
Pristine Talisman - Ramp you while ensuring you have a higher life total.
Wall of Shadows - infinite blocks
Hymn to Tourach
Faceless Butcher
Augur of Skulls
Chittering Rats
Gravedigger
Tendrils of Corruption
Ichor Slick
Geth's Verdict
Hideous End
Card draw/Deck manipulation:
Darksteel Pendant
Skulltap
Altar's Reap
Sign in Blood
Phyrexian Rager
Moriok Replica
Arcane Spyglass
Lose Hope
Fill with Fright
Casting of Bones
Long list of possible cantrips:
Afflict
Instill Infection
Expunge
Mind Stone
Gempalm Polluter
Chromatic Star
Cremate
Conjurer's Bauble
Dirge of Dread
Aphotic Wisps
Necrogen Spellbomb
Nighthaze
Gravebind
Headstone
Ichor Wellspring
Chromatic Sphere
Elsewhere Flask
Prophetic Prism
Recover
Relic of Progenitus
Sicken
Syphon Mind
Unburden
Unearth
Disciple of Malice
Backup win conditions:
Liliana's Shade
Twisted Abombination
Null Champion
Ulamog's Crusher
For other ideas that might fit, here's a mono-black Noxious Zombie deck I put together: http://tappedout.net/mtg-decks/noxious-ghoul-pauper-edh-1/
EDIT: Just thought this through, Aboshan's Desire gets pretty bad and stops protecting when you try to go "all in" and remove your graveyard to feed the 'Tog.
Convolute is fairly weak. I'd rather run the cheaper Mana Leak over it. In my blue Pauper EDH decks, I've been a big fan of Arcane Denial. It's a hard counter for 2 mana and its draw ability even plays well in your deck.
Stinkweed Imp could fit in well. Early chump blocker and its dredge can really fill up the graveyard, especially with all those looters.
Why are you running the artifact lands and Expedition Map? I don't see any interactions with them in your deck. Are you meaning to run Trinket Mage? That would work well.
The inclusion of Aboshan's Desire along with the obligatory Whispersilk Cloak is nice. Obviously you'll want some piece of mind knowing you won't lose everything when going all in with Psychatog. While it might not make the cut, the next best in this category is likely Mizzium Skin.
AEthersnipe is a Pauper EDH champ, leaving a beefy 4/4 body behind after bouncing a permanent. Recoil is likewise a well-rounded utility removal spell. Even if the deck weren't so blue, Drain Life and Consume Spirit are clunky, expensive removal spells that could probably stand to go. Snuff Out is a good contender.
Mind Stone is cheap ramp that also provides late game upside if you are flooding out.
EDIT: Presence of Gond is another great addition. Works alright as an untap target for Symbiote. Does a fair job boosting your cards that count elves. Also, it allows for infinite creatures if you stick it on Devoted Druid and have an Ivy Lane Denizen in play.