re-blue
/riːˈbluː/verb
1 [with object] To make blue again. rare.
2 [with object] US. To give (a gun) a fresh or further treatment for rust, damage, etc., using any of various processes which result in a blue-black protective finish.
Origin
Early 17th century; earliest use found in Joshua Sylvester (d. 1618), poet and translator. From re- + blue.