When can you Ninjitsu?

[sippingcider]

All ninjitsu says for when you can activate it is when you have an "unblocked attacker" to target. So obviously it has to be after declare attackers, but does "unblocked" require to be after declare blockers? Also, how long does the status "unblocked attacker" remain?

[Koby]

You can activate Ninjitsu in the following Combat Steps when you have priority:

1. Declare Blockers step. Note that the first action of this step is to declare blockers, and this does not use the stack.
2. Combat Damage step(s). Note that damage does not use the stack, and First Strike Combat Damage step is added when it's relevant.
3. End of Combat step.

[cdr]

http://www.yawgatog.com/resources/magic-rules/#unblocked_creature
Unblocked Creature: An attacking creature once no creature has been declared as a blocker for it, unless an effect has caused it to become blocked. It remains an unblocked creature until it's removed from combat or the combat phase ends, whichever comes first. See rule 509, "Declare Blockers Step."