i recommend (perma blue) is easy to handle for the beginners.
PROCEDURES:
1 sand out the original black coating to silver{as smooth as you can} by using sand papers
2 running water to clean and dry it by cloth
3 brush the bluing area(silver area) by brass brush and smooth it again
4 use brush to apply a layer of gun blue {must even}-note the effect will soon take place
5 running water and dry it by cloth and box tissue retouch
6 use brass brush to retouch slightly the bluing area (if necessary)
7 repeat steps 4 5 6