non on ne peut pas utiliser le ninjutsu durant l'étape de déclaration des attaquants car les bloqueurs n'étant pas encore déclarés, la réponse à la question "la créature attaquante est-elle bloquée ?" est indéterminée, on ne pourra y répondre par ou ou par non que lorsque les bloqueurs auront été déclarés à l'étape suivante de la phase de combat.
voir les règles suivantes :
702.48c A ninjutsu ability may be activated only while a creature on the battlefield is unblocked (see rule 509.1h). The creature with ninjutsu is put onto the battlefield unblocked. It will be attacking the same player or planeswalker as the creature that was returned to its owner’s hand.
509.1h An attacking creature with one or more creatures declared as blockers for it becomes a blocked creature; one with no creatures declared as blockers for it becomes an unblocked creature. This remains unchanged until the creature is removed from combat, an effect says that it becomes blocked or unblocked, or the combat phase ends, whichever comes first. A creature remains blocked even if all the creatures blocking it are removed from combat.