You choose the target creature or player as part of casting the spell. If that creature or player is an illegal target when Blast of Genius tries to resolve, it will be countered and none of its effects will happen. You won't draw or discard any cards.

If you discard a split card, Blast of Genius will deal damage equal the sum of its two converted mana costs as a single damage-dealing event.

If you discard a card with in its mana cost, the value of X is zero.