Pyroblast can target any spell or permanent, not just a blue one. It checks the color of the target only on resolution.
2016-06-08 • wotc
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.
2004-10-04 • wotc