Mono-Blue Terror's whole maindeck is blue, so Pyroblast is live against Counterspell, Brainstorm-fueled Delver of Secrets, and the Tolarian Terror/Cryptic Serpent threats themselves, making it a premium include. Hydroblast stays in the board since it only hits red permanents, which this deck has none of in its maindeck. We're racing their graveyard-discount threats, so trading our weakest noncreature slot for free counter-insurance keeps our clock intact without touching the artifact-based engine they can't interact with anyway.