This product is OK for touching up only. It's selenium based and is not the same chemical process as the various other blueing techniques (rust, nitride, caustic...) that are designed to give a tougher protective finish. My own experience with it is that it is very difficult to get an even finish and wears off very quickly.
Please also note that bluing steel retains the original surface finish - any imperfections in the surface will show through and sometimes even be highlighted by the bluing process.