Dimir Control has no red cards, so Hydroblast and Blue Elemental Blast have no targets and stay in the sideboard. Vandalblast could only hit a Baleful Strix or Mukotai Ambusher, so it stays out too. Dispel is strong insurance to protect your own Counterspell or key threat from their Counterspell/Dispel, and Arms of Hadar (-2/-2 to their creatures) can wipe their 1/1s (Baleful Strix, Spellstutter Sprite, Faerie Seer) if they overextend, though Augur of Bolas is a 1/3 and survives. Suffocating Fumes is largely redundant with Arms of Hadar and Thraben Charm is low-impact here, so both come out. This is a grindy mirror-ish matchup, so prioritize resolving card advantage engines like Mulldrifter and Murmuring Mystic while using Dispel to force through your own key spells.