There are graveyard decks that mill their own library, but these are rarely described as "mill decks." There is also some disagreement about whether decks that mill their opponents for value, like UB Rogues, count as "true" mill decks
You get another set of upkeep triggers
You can recur most of your combo pieces with Teshar or one of the retrieving creatures, and some of the available combo pieces can perform double duty as protection, too
Whats your favorite Treasure commander and why
The mono-Black sorcery five cost lets your foes choose things you don't control to destroy them
If you haven't noticed, monoblack can revolve a lot around using your own life as a resource, so I highly suggest playing cards such as Exquisite Blood and Gray Merchant of Asphodel to gain some of that resource back