There’s been a Mono-Black Eldrazi deck on the fringe of Standard recently, although it has yet to break through for a truly notable performance. Limiting yourself to one color when you want to cast colorless cards makes sense, since you’re essentially running a two-color manabase at that point, but with so few cards that actually require colorless mana and two lands (Aether Hub and Spire of Industry) that can serve as multicolor lands that also add colorless mana, I think there’s merit to adding a second color to help fill the deck’s holes.
Right now the Eldrazi decks are built around the power of Thought-Knot Seer and Reality Smasher. The problem is the early part of the curve isn’t powerful enough to keep up with Mardu Vehicles and G/B Aggro, and those heavy hitters aren’t nearly as good when you’re behind as when you’re ahead. What the deck needs is a card that can catch you up from behind and cement any games where you get out ahead.
Baral’s Expertise sounds perfect, then. Being able to clear your opponent’s battlefield and then land another creature, especially a Thought-Knot Seer, is incredible. To facilitate the added color and the higher curve, we find Cultivator’s Caravan added to the three-slot, and with Matter Reshaper and Scrapheap Scrounger in the deck, crew 3 is easy to achieve.
Having an accelerant is good anyway because you really need as much tempo as possible against G/B and Mardu. This one in particular plays nicely with Fatal Push, letting you cast a removal spell along with it on turn 3 so you’re not simply using the tempo advantage to recover from the disadvantage of essentially skipping your third turn.
The big advantage to playing this over the other aggressive decks in format is the number of utility lands. Ruins of Oran-Rief and Sea Gate Wreckage both help insulate you from flooding and the former is particularly good with Walking Ballista and recurred Scrapheap Scroungers. Knowing that you’re less likely to flood allows you to trade more aggressively and play to that late-game advantage, which is a nice default option to have. The card advantage from Matter Reshaper, Reality Smasher, Thought-Knot Seer, and Scrapheap Scrounger plays to this angle as well, and it’s always good to see the manabase play well strategically with the rest of the deck.
Given how resistant this Standard format has been to change, I doubt we’ll see this deck as anything more than the fringe archetype it has been, but that only plays to your advantage, taking people by surprise and unprepared for the matchup. Stale Standard formats breed players who spend all their time preparing for three to four decks, so the curveball is that much more effective. How we got to Eldrazi being the curveball is beyond me.