The "blocking phase" is actually known as the declare blockers step
. And anyway, that is a legal play. What happens is what you would expect. The unblocked creature becomes a 4/4 creature when Giant Growth
resolves, and you'll lose the game after it deals combat damage to you in the combat damage step.
However, the declare blockers step does not end until both players pass priority in succession while the stack is empty. So, if you have a spell or ability that can save you, you have ample time to use it. For example, Fog
to prevent the combat damage, or Murder
to destroy the unblocked creature, and so on.