SpoilerBy someone putting their hands on my threat, I am now aware of the attack, even if I can't see it. By attempting a grapple ... unless they are a grappling master (Improved Grapple), they just opened themselves up to an AOO. I don't know how you'd deal with shackles, but that can be a damned bloody weapon. I'll let you handle the appropriate penalties for not being able to see .... but I do have a tactile target, so I'll smash the stuffing out of the offending forearms.
By grabbing at her throat but being too clumsy to have gotten a hold on her yet, Necahual takes that split-second to make her assailant pay. She smashes one of her shackled wrists on her assailant's forearms with the well-crafted metal shackle .... maybe turning it a little so the hinge gets him for a real nasty gouge. And feeling the arms, she is not so unavailed as if against an entirely unseen opponent.
SpoilerStrike AOO: (1d20+6)[11]
Damage (1d20+3)[20]
Initiative for Next ... (1d20+1)[21]
EDIT: Well, I used the wrong die for damage, but no matter. I think I failed the attack roll, anyway.