Phillips Gun Blue is a cold-bluing paste made for touching up scratches and worn bluing on steel gunmetal.
The gel thickens on standing and turns more fluid again when shaken. Clean and degrease the area first, then work the paste in with a soft cloth in all directions until the bare steel darkens to match the surrounding bluing. Rinse off the residue with water once you're happy with the colour, then oil the metal well to protect the fresh finish.
It's a handy jar to keep in the cabinet for dealing with the odd chip or scuff picked up in the field.
As with any bluing chemical, work in a ventilated space, avoid skin contact, and keep it away from stock finishes and other components you don't want darkened.