I put some Birchwood-Casey Brass Black on a piece of highly polished brass and after several coats it darkened the brass to a very attractive shiny bluish color. Not antiqued at all but I like it a lot and I've left it like that.
I took another piece and suspended it over ammonia in a closed container and let the fumes get to it. That eventually caused an antique appearing patina on the brass. That looks more random and authentic.
Suspend the pieces in a jar above an inch or so of vinegar, not in the vinegar, it is the gas off that will turn the brass. I make a few air holes in the lid, suspension wire through lid and let the piece hang, check every couple days, Ammonia is faster, but if you let it go too long, it won't be pleasant.... Lightly wipe the piece with 3x steel wool and let it go again. It will look more natural (and it is) than staining. This works, I have used it for many years. Just be sure to remove any "finish" that may have been applied to the brass.