Protection from a color (or any quality) grants four specific immunities, remembered by the acronym DEBT: the creature cannot be Damaged, Enchanted/Equipped, Blocked, or Targeted by anything that has that quality. Since Unsummon is a blue instant that says "Return target creature to its owner's hand," it requires choosing a legal target (CR 114.9), and a creature with protection from blue cannot be targeted by blue spells or abilities (CR 702.16b).
If you attempt to cast Unsummon and choose a creature with protection from blue as the target, the target is illegal. If the protection was already in place when you tried to target it, you simply cannot choose it as a target at all — the spell can't even be cast targeting that creature.
If somehow the creature gained protection from blue after Unsummon was already on the stack (e.g., its controller activated an ability in response), the spell would be countered on resolution because its only target is now illegal (CR 608.2b).
Concrete example: Your opponent controls a Beloved Princess that has been given protection from blue via an aura. You cast Unsummon targeting it — this is illegal. You cannot choose that creature as a target. If the Princess gained protection in response to your Unsummon, the spell resolves but is countered because it has no legal targets remaining.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.