Can enchantments (briefly) go on protected creatures?
The player must choose a legal object or player according to the Aura’s enchant ability and any other applicable effects.


Now, what is a legal object? That isn't explicitly defined, and neither is illegal object:


  303.4c If an Aura is enchanting an illegal object or player as defined by its enchant ability and other applicable effects, the object it was attached to no longer exists, or the player it was attached to has left the game, the Aura is put into its owner’s graveyard. (This is a state-based action. See rule 704.)


However, the same word legally is used in


  702.16n Some Auras both give the enchanted creature protection and say “this effect doesn’t remove” either that specific Aura or all Auras. This means that the specified Auras can legally enchant that creature and aren’t put into their owners’ graveyards as a state-based action. If the creature has other instances of protection from the same quality, those instances affect Auras as normal.


which would lead me to conclude that a creature with protection from blue is an illegal target destination for a flickered Reality Acid.