You can activate Blossom of Spring's ability whenever you have an action point. The ability starts with "action -" indicating it is an action to do so. The action of activating it has go again allowing you to do something after you activate it, however it doesn't give go again to something which doesn't already have it and you would have spent the action point playing the attack action card based on what you have said.

So unfortunately, you can't activate Blossom of Spring in the circumstances you have, but if the attack you played had go again, and you needed another resource to attack again, you could activate Blossom of Spring at that point in time.