You will unlock the poison Blade a bit after Sequence 4 and are given 5 vials of poison to use.
and select the poison Blade (To the left of the double blade) now find a guard that is near a raised platform, I found this was easier on the roof tops as I could climb on a chimney after poisoning them.
Walk up and press
to administer the poison then quickly climb something near them and equip your double blade by pressing
on the Dpad and press
to target the now poisoned guard then press
to assassinate them.
Heres a video by SuperDriver101