Can I use Moonlace to allow a Lightning Bolt to target a creature with protection from red?
Moonlace only works on Lightning Bolt when it's a spell, i.e. an object on the stack. To get it there, you have to cast it, which includes specifying a legal target. As you indicated, Repentant Blacksmith isn't one.

What would work is casting the Lightning Bolt on another target (e.g. your opponent), Moonlacing it so it becomes colorless, and then change the target with another card, e.g. Misdirection.