In contrast to humans, cats can use their 3rd eye lids to blink. It basically looks as if they pulled a thin membrane over their eye and it's something you most likely won't ever see happen.
(If you do ever see it, you'll probably freak out)
Thus if a cat actual does a human blink, by squinting, it's usually on purpose. It's basically a way to say "I trust you," since they're making themselves slightly vulnerable.
Ps: Humans also have a third eyelid. Ours is however useless.