Blind is arguably one of the strongest spells in the game. It is perhaps at its best combined with Resurrection spell, and managing to blind opponent's last creature stack. This leads to a situation where the opponent will have no chance to retreat, and the hero can cast Resurrection to bring many if not all his troops back to life, provided enough spell power and mana. This tactics weak spot is War Machines, which cannot be blinded and therefore with Artillery or First Aid secondary skill may offer enemy hero a chance to escape. Additionaly, during siege battles Ballistics secondary skill may offer a chance for the enemy hero to escape.
The only drawback of Blind is, that there are many creatures immune to it. Solution to this problem is to find a relic artifact Orb of Vulnerability, which disables spell immunities and enables hero to blind even naturally magic immune, e.g. Black Dragons.