Spell Blast

• WWizards of the Coast2013-07-01
  Official Ruling
  If the target spell has in its mana cost, include the value chosen for that X when determining the value of the in Spell Blast's mana cost. For example, if you wish to counter Volcanic Geyser (a spell with mana cost ) whose X is 7, you'll need to choose 9 for Spell Blast's X and pay .
• WWizards of the Coast2013-07-01
  Official Ruling
  Alternative costs, additional costs, cost increases, and cost reductions don't affect a spell's mana value.

Rulings via Scryfall