All blocking creatures are declared at the very beginning of the declaration of blockers step.  In order for a creature to be declared as a blocker it must be in play when this step begins.  This means that the last chance you have to either get a creature into the battlefield or make another permanent into a creature so it can block is in the declaration of attackers step after attackers have been declared.

Keep in mind that it you do animate a permanent or cast a creature as an instant, your opponent can still respond before the declaration of blockers step begins and destroy this creature so it cannot block.  This is because the game does not advance unless all players pass priority on an empty stack.