You don't need an element, you need a mineral.
Certain elements do tend to make things a certain colour, for example nickel will make minerals green, manganese pink, and cobalt purple.
The fact that cobalt itself is a metal is irrelevant. Whatever "element" you have will not be metallic, it will be as a cation (or anion) in the silicates.
So your question should be rephrased to:
What element do I need to make the silicates the moon is made of, blue?
You have several options.
- Add sodium and chlorine, in order to make sodalite: Na8Al6Si6O24Cl2 (with the other elements already abundantly present on the moon).
- Sodium and water, at high pressure and then somehow expose those rocks on the surface. You will have glaucophane: Na2(Mg3Al2)Si8O22(OH)2. This is the main ingredient of the terrestrial rocks known as blueschists:
- Tons more aluminium, so then you can stabilise corundum (Al2O3). When combined with the already abundant iron and titanium as trace elements, you end up with blue corundum. Also known as sapphire when in gem quality:
This should get you started. Other things you might consider are potassium and whatever makes amazonite (potassium feldspar) green-blue. Copper is also a good one, but the blue requires water that isn't common on the Moon.