I swap cards in from my side board when I decide to play non-duel commander.

I would focus just a tad more on unblockable effects.

FYI: I did get shut down by an icy manipulator recently. Do you have any suggestions for un unblockable stuff?

Prowler's Helm could be a possibility, i mained that for a tournament and won due to it, and that's precisely why i run walls in my decks XD

This looks like a great deck, definitely something I would play. Maybe Hand of the Praetors to make skithiryx bigger and have more infectors. Speaking of, Plague Stinger, Ichorclaw Myr, Necroskitter, Midnight Banshee, and Ichor Rats are pretty good black infect creatures. As far as none creature support goes, Tainted Strike, Glistening Oil, Phyresis, and Contagion Engine are a must. I hope that these are helpful to you.

Here's my 1v1 skithiryx list: http://tappedout.net/mtg-decks/blight-dragon-4/ This deck is pretty brutal, it can often deal with 3 players like an "archenemy" deck.

Skithiryx is all about the math, but it's not about counting to 10. It's about counting to 3 and trying to reduce 3 to 2 to 1. 3 is the number of hits that the blight dragon needs to connect with to kill someone. What are the most efficient ways to reduce that 3 number to 2 or 1? To reduce 3 to 2 for 0 mana on the same turn, use any one of: Shuko, Cathedral of War, Power Matrix, or unspeakable symbol. If you miss giving him +1 power on his first attack you need to give him +2 or power on his second attack. any other number is just inefficient. So use Funeral Charm or a +2 power equipment. To reduce his kill timer to 1 round, and kill in one hit on the turn he comes into play, you need to give him +6 power for no mana, so use cards like Lashwrithe, nightmare lash, and Liliana of the Dark Realms.

If you hit with the dragon once and he dies, let him "die" and you can cheat commander tax and bring him back for the instant kill with Fearsome Awakening. Remember that reanimating Skittles will often be cheaper than recasting him with the commander tax if you have a Necromancy or even a Zombify.

Shuko, Cathedral of War, Power Matrix, Unspeakable Symbol, Necromancy.

To deal with Icy manipulator and maze of ith: Pithing Needle, Lightning Greaves, Surestrike Trident. The trident even gets around things like Peacekeeper and Stonehorn Dignitary.

I would make room for Livewire Lash and Liliana of the Dark Realms. Also, Nameless Inversion could double as both removal for opponents' stuff and pump for your gen.

Mono black can afford to run Strip Mine, Wasteland, Tectonic Edge to deal with an opponent's Maze of Ith or Mystifying Maze, which can otherwise shut you down pretty efficiently.

Maybe even Goryo's Vengeance, to get Skittles out for cheap if/when he dies?

I don't like Dark Confidant or Dark Tutelage in a deck that tops out at 12CMC. The likelihood is low, but why take the risk when you have access to Necropotence & Yawgmoth's Bargain?

booflesnazzer yawgmoth's bargain is banned in commander, otherwise i agree about the last thing you said. Phyrexian Arena is good too if you dont have it already

All the swords. I don't want to list them all but here's a couple: Sword of Light and Shadow Sword of Fire and Ice.

Dauthi Embrace