Do ya want a black gun or just a no-shine finish?
Do you have a stainless (K) or blued (RB) gun??
Yes, you can get yer stainless six-gun blackened with a phosphate-oxide finish, I recollect that's the right process. Any 'smith who does bluing could answer jus' 'zactly what the process is called. He might not be able to do it at his shop, though.
If ya have a RB (blued) Ruger six-gun & ya want it flat black---Ya bead blast the finish & the blue it without polishing. Many 'smiths can do that jus' fine.
MG