Personally my favorite casual mono-black commander has been Kuon, Ogre Ascendant. He is good in devotion builds for nykthos, even better now that we have 4x fleshbag marauders, great at deflecting early aggro, and an excellent distraction from the command zone.
Links to my most current deck lists;
Primary EDH; Rakka Mar Token Perfection, Crosis Mnemonic Betrayal, Cromat Villainous, Judith Gravestorm, Rakdos Empty Storm, Exava Artifacts, Bant Trash, & Fumiko Voltron!
EDH kept at home; Ruzzian Isset & Rakdos LoR!
EDH (nostalgic/pimp/retired) in storage;
Latulla Burns, Akroma Smash, Jeska Voltron, Rakdos Storm, Bladewing Darghans, Lyzolda Worldgorger, Xantcha Steals your Heart, Jori Storm, Wydwen Permission, Gwendlyn Paradox, Jeleva Warps, & Sigarda Brick!
Legacy Showanimator and High Tide!