Call it the thinking mage's version of brute force.

Spell Blast

Counter target spell with mana value X. (For example, if that spell's mana cost is {3}{U}{U}, X is 5.)

30th Anniversary Edition

Tempest Remastered

Counter target spell with converted mana cost X.

"Perhaps you could try throwing a rock? That might be better suited to your intellect."

Magic 2014

Counter target spell with converted mana cost X. (For example, if that spell's mana cost is {3}{U}{U}, X is 5.)

Classic Sixth Edition

Counter target spell with converted mana cost equal to X.

Alternative costs, additional costs, cost increases, and cost reductions don't affect a spell's mana value.

If the target spell has {X} in its mana cost, include the value chosen for that X when determining the value of the {X} in Spell Blast's mana cost. For example, if you wish to counter Volcanic Geyser (a spell with mana cost {X}{R}{R}) whose X is 7, you'll need to choose 9 for Spell Blast's X and pay {9}{U}.