Other iron rule: never pull shit like that on your actors. Be prepared extends to let them be prepared too.
Genuine and unscripted reactions are best when they come up organically from them. If they're suddenly overwhelmed and cry, forget their lines, adlib — then, you let it rule and see if you struck gold.
Surprise, on top of being unethical if not outright problematic, is also overrated. Knowing what you're going to have to act doesn't take anything of what you'll feel actually doing it.
Especially with this blindfold setup, she would have reacted genuinely to the touch even with the heads-up!