Do copper pipes corrode with hard water. Mine have been in the house for alamost thirty years and are turning blue about th joints. If this happens do they need to be replaced. If so, how expensive (approximately)would it be. Our house is two stores and 3,000 sq ft. Also, are there copper pipes in the walls? Thank You.

If you had corrosion you would ne getting pin holes in the pipes, not at the joints.
Solder flux will leave a greenish blue tint around the joints and that is what I think you're seeing.

are there copper pipes in the walls? Yes! Slab homes haveall the manifolds in the walls and in your two story all the branches leading upstairs are copper as are the separate supplies to each fixture. All are in your walls.
I think your worries are needless. IMO you aren't ready for a repipe job just yet. Good luck, Tom