Blast of Genius — ruling 1

If the target permanent or player becomes an illegal target for Blast of Genius, the spell doesn’t resolve. You don’t draw or discard any cards.

Blast of Genius — ruling 2

If a discarded card has {X} in its mana cost, X is considered to be 0.

Blast of Genius : règles, fonctionnement et interactions (MTG)

Blast of Genius is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Choose any target. Draw three cards, then discard a card. Blast of Genius deals damage equal to the discarded card's mana value to that permanent or player.

How it works : Blast of Genius

Original English text — Wizards of the Coast / Scryfall

#1December 7, 2018· Wizards of the Coast
If the target permanent or player becomes an illegal target for Blast of Genius, the spell doesn’t resolve. You don’t draw or discard any cards.
No other card is named in this ruling.
#2December 7, 2018· Wizards of the Coast
If a discarded card has in its mana cost, X is considered to be 0.
No other card is named in this ruling.