Pyroblast stays in the sideboard: Mono Red Madness only plays Mountains, so its one blue card, Sneaky Snacker, never gets cast, and Pyroblast has no legal target that matters. Cast into the Fire deals 1 damage to each of up to two creatures, which is enough to kill 1-toughness creatures like Voldaren Epicure (and a Sneaky Snacker if one is on the battlefield) but not Kessig Flamebreather or Guttersnipe on their own. It is still a cheap way to trade with their small creatures and slow the chip damage, and its exile mode can remove an artifact such as a Blood token. Weather the Storm buys life to survive a critical burn turn, especially once Fireblast and Fiery Temper enter the picture. Structural Distortion and both Sagu Wildlings are the weakest cards against a deck with few relevant artifacts to exile and no profitable combat for a 3/3 flier to answer.

Dimir Control plays no red cards, so Blue Elemental Blast and Hydroblast are the cards to watch from their side, but they don't threaten our colorless Eldrazi. Their deck is full of blue cards (Counterspell, Brainstorm, Spellstutter Sprite, Ninja of the Deep Hours, Baleful Strix), so Pyroblast has plenty of legal targets. It can counter a blue spell such as Counterspell to protect a key threat, or destroy a blue permanent such as Spellstutter Sprite, Ninja of the Deep Hours or Baleful Strix. Bring in Pyroblast in place of the slow Bonder's Ornament and the Structural Distortions, and be mindful of our limited red sources. Their only artifacts are artifact creatures (Baleful Strix, Mukotai Ambusher). Vandalblast could destroy those, but that is a thin target pool, and Gorilla Shaman only hits noncreature artifacts, so it has no targets here and should stay boarded out. Relic of Progenitus doesn't meaningfully slow down their card advantage engine (Lorien Revealed, Brainstorm, Ninja of the Deep Hours) since those aren't graveyard-dependent. Breath Weapon deals 2 damage to each non-Dragon creature, so it would kill their 1/1s (Baleful Strix, Spellstutter Sprite, Faerie Seer) and also their 2-toughness creatures like Ninja of the Deep Hours and Mukotai Ambusher. It also kills our own Arbor Elves, though, and it can be countered, so it isn't worth the slot over a counterspell-resistant big creature plan. Otherwise, play around counterspells and removal by sequencing threats and using cascade triggers to rebuild card advantage.

Mono-Blue Terror's maindeck is entirely blue, so Pyroblast counters their Counterspell, Brainstorm, or removes a resolved Tolarian Terror/Cryptic Serpent/Delver of Secrets, making it one of the best sideboard cards available. Hydroblast and Blue Elemental Blast are already excluded since they only hit red, which this opponent isn't playing — that's correctly reflected by not bringing those in. Gorilla Shaman has no artifacts to target here, but it's a passable 1/1 body; more importantly the Pyroblasts are the priority swap. Structural Distortion and extra Sagu Wildlings come out since they do nothing proactive against a tempo-mill shell that wants to race or counter you, not fight over artifacts or lands.