Assassinate and backstab require the attacker to be sneaking or invisible, unlike sneak attack. Lets say the victim is blinded or paralyzed, would that trigger assassinate and backstab as if the attacker was invisible?

Assassinate and backstab will only work if you are sneaking/invisible, no exceptions.

I'd like to see more options to trigger backstabs especially since the 1.1 nerf, but allowing them on enemies who can be paralyzed sometimes upwards of 10 seconds (and blinded for far longer) would be game breaking. That said, there is some good gear that allows assassins to capitalize on backstabs (a cloak that adds stun to any attack made from stealth, new boots that grant a free per-encounter invisibility after a kill, etc) and adds some helpful utility to stealth builds.