For personal reasons, all of my builds are mono-color.  There are other mono-black Pox builds (and many that splash other colors, particularly green), but I like this one because of its symmetry.  8 cards drop lands.  8 drop cards from hand.  8 drop creatures.  8 drop all of the above.  8 are creatures that cheat out fast.  The redundancies make for more consistent play, and there is obviously some very strong synergy.  The 19 lands and one odd card is my solution to the fact that I like blocks of four (for the consisnt play I mentioned above), but a fifteen sideboard is not a multiple of 4.  Dark ritual when it needs to be faster.  Pithing needles for manlands and some combo decks.  Extirpate for combo decks.  Innocent blood for aggro decks.  I have run engineered plague, particular in meta-games where tribal decks are popular.  This deck hates spell-snare (count all the 2-cast!)