Any life gain deck with a chance of accessing B should have at LEAST:
1 Yawgmoth's Bargain. GOLD in these decks once you mid-game.
1 Necropotence. Again...Gold. You have the manabase to support it.
X Greed. Put that life gain to good use.
Just my opinion...but when you add B to lifegain, you must add some pay life/draw that black offers. If in black, life=cards :).
Also, the Flats can act as a 5-8th copy of Godless Shrine. Yeah, it costs 3 life to put it into play untapped, but with a deck focusing on life gain, it shouldn't be a problem.
Also, thoughts on Serra Ascendant? He can act as another finisher, and he is a one drop.
-1 Knight of Meadowgrain(2 :W: isn't bad, but I figure you could use some more cheaper drops.)
-1 Martyr of Sands(Meh, I don't think she's bad, but I would rather have some aggressor that can become bigger late game, than possibly drawing into someone who only nets you more life.)
-1 Vampire Nighthawk(Again, just to make room for cheaper dudes.)
+3 Serra Ascendant(One drop life linker, and can become really big once you get enough life.)
No Sanguine Bond? Dropping some of the higher end stuff like divinity of pride and baneslayer will help your mana curve. It's already been suggested, but check out serra ascendant and maybe the Soul Sisters deck in general for a few more ideas.
Martyr of sands is weaker here with mono-black cards in the deck, which limits the life you can gain with it.
Bargain and co certainly aren't bad additions (greed is debatable), but Sign in Blood or something similar is a better place to start.
I used to run Sanguine Bond in place of Baneslayer Angel. The problem was, if I got into a lock with an opponent, I wasn't able to stall and folded. An untapped BSA combined with the rest of the deck lets me stall forever. By the time I resolve whatever the problem is I've got 50+ life and enough room to fool around a bit.
As for Serra Ascendant... I'm just not sure what to take out. If anything, probably Soul Warden. The Knights, Vampires and Martyrs gain a nice amount of life early game. I'll have to do some MWS testing with Serra Ascendant.
Also, Martyr's not weak at all with only eight mono-black cards. I typically get 9-12 life out of one Martyr early game.