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Did you ever consider Exotic Orchard for card:Praetor's Grasp? I think it'd be perfect, and doesn't require the extra 1 mana like Shimmering Grotto does to produce the color you need. Surely your opponent's lands produce the color(s) required for his own spells. I run four in my blue theft deck.
I love your deck, and I hope these humble suggestions prove useful in some way.
+2 Liliana of the Dark Realms ->> Initially I was not a fan of her, but I have grown to love her. She's a great distraction, a great mana fixer with shockswamps, and she helps you with your lifelink/swamp combo. Her ultimate doesn't synergize with your deck, but maybe there is an XB card out there to consider?
Mana fixing with Lili to help you cast cards you steal.
Best card in black, period.
If you can run with shockswamps, you don't need this. You probably have enough problems with the colorless mana sources and BBB spells.
-3 Dark Favor
Seems like you are inviting card disadvantage, but when played on a lifelink flyer, it's probably decent value if you can connect with it just once.
22 Land seems low for a deck that runs at needs to hit 3 on 3, 24-25 seems right to me.
Finally, another card that came to mind was Hypnotic Specter. Good job so far, +1 from me. :)