Modern is a format where the manabases are strong enough that most decks play lots of colors unless they are explicitly trying to take advantage of opposing manabases with a card like Blood Moon. But in such a large format, there is the potential to play lots of utility lands if you limit yourself on colors, and those utility lands are quite powerful. Today's list has a lot of those lands while still retaining a powerful tempo shell in a single color.
Field of Ruin is the real star, and before the unbans it was among the hottest cards in the format. It has dropped off somewhat since Bloodbraid Elf's return, but the card is still quite good. It cleanly answers Tron, creature-lands, and things like Cavern of Souls that are a bane for decks that rely on counterspells.
Faerie Conclave is the less exciting of the two, but for a deck like this. it's still valuable. The goal here is to tread water in the early game before turning the corner with Torrential Gearhulk or Elder Deep-Fiend and once you turn the corner, getting in for two or four damage can mean the difference between winning the race and losing it or giving your opponent an extra turn to draw a lethal burn spell.
The last two utility lands, Academy Ruins and Oboro, Palace of the Clouds, are also minor, but significant upgrades. Academy Ruins works nicely with Torrential Gearhulk and various sideboard cards while Oboro lets you filter colorless mana from Field of Ruin into blue mana should you need to cast something on your turn and leave up a card like Cryptic Command. It's not much, but every little bit counts.
The main sacrifice in moving to mono-blue is the weakness of blue's removal. Dismember is good but playing three copies gives back the life you gained from not playing fetch and shock lands, though it does work nicely with Torrential Gearhulk. Piracy Charm is even more questionable, but having a clean answer to mana creatures, Thalia, Guardian of Thraben, and Young Pyromancer is important. Piracy Charm isn't flashy, but it gets the job done, and that's what you're looking for when your card pool is limited.
I was immediately drawn to the utility lands here, but it's possible that the star of the show is Torrential Gearhulk. Recasting Cryptic Command with it is absolutely filthy, even in comparison to the powerful things going on in Modern, and the card dodges most non-Path to Exile removal in the format. We've seen it make its way into Jeskai lists as a singleton, but this deck does a better job of taking advantage of the 5/6 body.
Since the unbans the hype in Modern is around attrition decks, but Magic is about getting your opponent dead, and that's what Torrential Gearhulk likes to do. Why fight it?