A double-strength blueing solution designed for blueing highly polished steel parts and hardened steels containing nickel and chrome alloys (except stainless). Super Blue Extra Strength Gun Blue is the blackest blue. Use caution to confine the solution to the area to be reblued, as it will have an effect on the existing blue. Double-strength blueing solution. For blueing highly polished steel parts and hardened steels containing nickel and chrome alloys (except stainless). Super Blue Extra Strength Gun Blue is the blackest blue.