If the blackened layer does not last, there are several options.
1. If the surface is nano-coated or covered with an invisible clear varnish, this layer must be removed so that the bluing liquid can adhere.
2. The surface may be made of an incompatible material.
3. The pre-treatment may not have been carried out thoroughly enough. Bluing will only work on a clean surface, free of dust, dirt and other contaminants.
4. The bluing agent may have been too concentrated. Depending on the product, you may dare to dilute it with water.