The hippo grows wings to fight the condor.
—Nantuko teaching

Seedtime

Cast this spell only during your turn.
Take an extra turn after this one if an opponent cast a blue spell this turn.

Mystery Booster 2

Judgment

Play Seedtime only during your turn.
Take an extra turn after this one if an opponent played a blue spell this turn.

You only get an extra turn if an opponent cast a blue spell this turn prior to this spell resolving. Note that "casting" a spell only requires it to be announced, so an unresolved spell still counts as having been cast.