“Mono-Blue Terror is a tempo-combo hybrid that uses cheap self-mill spells (Thought Scour, Mental Note) to rapidly fill the graveyard, enabling Tolarian Terror and Cryptic Serpent to hit the board as massive threats for very little mana. The deck pairs these threats with a permission suite headlined by Counterspell and Force Spike to protect its board and disrupt the opponent while Delver of Secrets provides an additional early clock. The engine lives and dies on graveyard density, so the mill-cantrip package and the cost-reduction threats are inseparable and must be preserved at all costs. Its maindeck is entirely blue and plays no artifacts; its only red card is a sideboard Gut Shot (Phyrexian mana, usually paid with life). So Hydroblast and Blue Elemental Blast have no real targets against it and artifact hate (Steel Sabotage, Dust to Dust) has nothing to hit, while Pyroblast and Red Elemental Blast hit nearly every card it plays.”
Missing sideboard cards? The full 15-card sideboard runs about $44.13.
Buy sideboard on ManapoolHydroblast is excellent here, countering or destroying Fiery Temper, Lightning Bolt, Lava Dart, Kessig Flamebreather, Guttersnipe and Voldaren Epicure, all of which are red. Gut Shot deals only 1 damage, so it is a narrow answer: it kills 1-toughness creatures like Voldaren Epicure and Sneaky Snacker, but it will not kill Guttersnipe (2/2) or Kessig Flamebreather (1/3). Epicure's damage trigger happens as it enters, so Gut Shot cannot prevent it. Since you have no red mana, each Gut Shot costs 2 life, which matters against a burn deck. Spell Pierce is weak since the opponent is rarely tapped out early, and Sleep of the Dead and Deem Inferior are too slow against a deck trying to race with direct damage. Keep the Counterspell suite and the remaining Dispels to protect your life total and your graveyard-fueled threats.
Dimir Control has no red cards, so Hydroblast and Blue Elemental Blast stay in the sideboard, and its only artifacts are Baleful Strix and Mukotai Ambusher, making Steel Sabotage narrow but live as a cheap way to counter or bounce Strix before it replaces itself. This is a grindy mirror-ish matchup where card advantage and resolving threats matter more than tempo tricks, so Deem Inferior is the easiest cut since it's weak against a deck that doesn't flood the board with must-answer permanents early. Keep Counterspell, Dispel and the full mill-cantrip package intact to keep casting cheap Tolarian Terrors and Cryptic Serpents under their countermagic.
Spy Combo is a green-black ramp/ritual deck with no red or artifact permanents worth naming (its artifacts are only incidental, like the opponent's own Food/mana rocks are not present here), so Hydroblast, Blue Elemental Blast, Annul and Steel Sabotage have no legal targets. Pyroblast and Red Elemental Blast are not in our sideboard, and none of our other sideboard cards (Gut Shot targets anything but is a marginal upgrade) meaningfully improve this matchup over our core plan. The best approach is simply to race and counter their key enablers (Winding Way, Lead the Stampede, Dread Return) with the existing Counterspell/Dispel suite while our cheap Terrors and Serpents close the game before their go-wide plan comes online. No sideboarding is recommended since nothing in the sideboard clearly upgrades over the 60 cards already in the deck.