how to blind a robot and not the human?
It depends on what do you expect and what this would be for.
I've mentioned the Captcha solution for miniBB above. It's very customizable. You can design and use your own fonts, which would be recognizable by a human, but would become a tricky task for a robot. You can set a difficult-to-guess secret key, which is impossible to de-cypher. You can set the Captcha to ask for entering more symbols (let's say 8 symbols are almost not guessable by a robot).
By default it comes with a very basic fonts set. So far I know some cases, when miniBB is being spammed by "Chinese humans" which could break any robot. But this is manual spam. The Captcha version we have, could be called an "absolute balance" between the human abilities and robotism. So far I can't recommend a better solution than this.
You know, spamming always grows, as the technologies grow. Whatever you would invent, could be broken; if broken not in a day, then in a week or so. So the task is not about "how to blind a robot and not a human
", because you can't blind a robot...