Obligatory quibble: Nobody is “blocked” in robots.txt. They are “denied” or “disallowed”. It’s the difference between installing a deadbolt and putting up a “No Admittance” sign. With compliant robots it should make no difference--yandex is currently compliant--but it’s important to keep a firm grip on the difference.
robots.txt is a guideline that politely asks bots to not visit. The good ones, Yandex is a good one, should follow the guideline. Most other bots completely ignore robots.txt. In fact most bots don't even read the robots.txt. For bots other than the good ones, expect the robots.txt to be completely ineffective.