It depends on the age of the Sheridan. Most recent versions of the rifles have been painted. For small touch-ups on my 392 (.22 version of the current Blue Streak) I've used flat black acrylic craft paint. Almost perfect match and nearly as durable as the original paint. Which isn't saying much for the durabilty of the original paint.
The older Sheridans were chemically blackened with something called Ebonal-C, a chemical blackener. I don't think you can blue brass/bronze very well. I tried some Birchwood Casey blue on some polished brass and it ended up looking more like color case hardening than the traditional blue/black look of blued steel.
Birchwood Casey (maybe other companies too) makes a product called Brass Black that will chemically blacken brass. If I wanted to touch up my '65 Dan that is what I would try. But mine's not too scarred up and I like the "patina" that comes with nearly 50 years of service.
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