Its a good deck, but I would change some things: I don't like Rack, IMO, it's very very slow, I would put some creature as 'Bloodghast' in place, even more because you have fetchlands. Try to fit some Gatekeeper of Malakir in deck, or Side, i love them. I'll move Doom Blade to Side, and put Go for the throat. And change Victim of Night for Smother or Diabolic edict. And again, Sry for grammatical errors, I hope it to understand.
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1 - Bob is one of the best black cards in magic. Yes it can kill you at any time, but that's why he put High Market and Phyrexian Tower to kill Bob when necessary. The price to pay for draw 2 cards per turn is fair. 2 - Fecth Lands (in this case, the Mash Flats), is a great card to shuffle the deck, and at the same time get a land. It is good to avoid bad luck. 3 - Seize another extremely strong black card. The cost is to balance its optimal effect, I think extremely fair as well. An option to Seize, is Inquisition of Kozilek, she is weaker than Seize, but you don't lose life. Or Duress, She is weaker than Kozilek, but without restriction of cost. 4 - Urbog doesn't change your land, only adds a type. For Example, Transform an Island in Island&Swamp. And, Sry about bad english, if have a lot of grammatical errors, isn't my native language.