Choose one —
• Counter target spell if it's blue.
• Destroy target permanent if it's blue.
Pyroblast can target any spell or permanent, not just a blue one. It checks the color of the target only on resolution.
The decision to counter a spell or destroy a permanent is a decision made on announcement before the target is selected. If the spell is redirected, this mode can’t be changed, so only targets of the selected type are valid.