can emrakul be blocked?

It may be blocked. If it isn't blocked, it simply won't deal combat damage to anything. Emrakul's second ability triggers as you cast it, and that ability resolves before the spell itself. It resolves even if that spell is somehow removed from the stack.

• Exile it with Oblivion Ring.
• Destroy it with Executioner's Capsule, or Avatar of Woe.
• Destroy it with Supreme Verdict, or put it on the bottom of its owner's library with Terminus.
• Return it to it's owner's hand with Venser, Shaper Savant.