If a creature is given protection from all colors after blockers are declared does it become unblocked?

Protection prevents four things, often remembered by the acronym DEBT: the creature cannot be Damaged, Enchanted/Equipped, Blocked, or Targeted by sources with the relevant quality. The 'can't be blocked' aspect of protection only matters at the time blockers are declared (CR 509.1b). Once a legal block has been established, it stays established.

According to CR 702.16k, protection does not retroactively undo a block that was already legally declared. The block was legal at the moment it was made, so the creatures remain legally blocking and blocked. Nothing in the rules causes a creature to become unblocked simply because it gains protection after the blocking step.

CR 509.5 confirms that a creature remains blocked even if the blocking creature is later removed from combat. Gaining protection is not the same as removing the blocker, but the principle reinforces that blocking status is locked in once assigned — it takes a specific effect that says 'creature becomes unblocked' to change that status.

Concrete example: Your 3/3 attacks and your opponent blocks it with a 2/2. You then cast a spell giving your 3/3 protection from all colors. The 3/3 is still blocked — combat damage will not be dealt to your opponent, because the 2/2 is still the blocker. However, the 2/2 cannot deal damage to the 3/3 (protection prevents damage from the colored source), so the 3/3 takes no damage while the 2/2 takes 3 damage.

In summary: gaining protection after blockers are declared does not make the protected creature unblocked. It does, however, prevent the blocking creature (assuming it is a colored source) from dealing damage to it during the damage step.