Mezzio Mugger

You don't have to attack with the creature with blitz unless another ability says you do.

If you choose to pay the blitz cost rather than the mana cost, you're still casting the spell. It goes on the stack and can be responded to and countered. You can cast a creature spell for its blitz cost only if you could cast that creature spell. Most of the time, this means during your main phase when the stack is empty.

You must follow all normal timing rules for cards played this way. For example, you may play lands this way only during your main phase while no spells or abilities are on the stack and only if you haven't played a land yet this turn.