There are various "Sight Black" sprays on the market, and I'm assuming you just use a brief shot of it on the sight. An old (and cheap) trick is to simply use a lighter or a match, and hold the flame on the sight for a few seconds. Since the sight is metal, it won't be damaged, but the flame deposits a thin layer of soot.
The idea is to get a nice, flat black surface, which makes the sight stand out more against the target.