The main attraction to Mono-Black Infect is gaining Phyrexian Crusader. This guy dodges everything from Lightning Bolt to Path to Exile. Heck, even Murderous Redcap can't target him. Much of the removal going around is limited to non-black creatures as well. Neither Slaughter Pact nor Shriekmaw can touch black Infect creatures.

The Thoughtseizes and Inquisition of Kozileks take the place of the reactive spells that the other Infect lists play: Apostle's Blessing and Vines of Vastwood. Gitaxian Probe has been a boon for Infect for a long time to know when the coast is clear to go for the win, but with the discard spells to see their hand you get to preemptively gain information while clearing out any troublesome removal spell.

Mono-Black Infect goes for more of a long game approach than the others and is incapable of winning before turn 4, and even that is a longshot as Pendelhaven, Cathedral of War, Runechanter's Pike, and Funeral Charm are the only ways to increase the power of your creatures. The deck plays a much more fair game while being more resilient and interactive to what your opponent is doing.