If you try the ammonia method, do not put the ammonia solution directly on the part.
You will need to throughly degrease/dewax the parts. Boiling them in water will do a good job if you just let them cook for a while.
After degreasing the parts you will need to make some sort of closed container.
If you have a glass jar with a lid that is large enough to hold the parts your off to a good start. Also get a piece of wire.
Poke two small holes for a piece of wire thru the lid and suspend the parts from the wire.
Put a little ammonia in the jar, lower the parts into it and screw on the lid.
This takes some time so sit back or go to bed and wait till morning.
If you used a glass jar, take a look at the parts. When they are dark enough to suit you your done.
A word of warning about doing this to parts which will be subject to stress.
Ammonia has been known to embrittle brass and bronze so don't try this on a brass gun barrel or breech plug.
The finish, like Birchwood Caseys Brass Black is just a thin surface finish and will show wear if it gets rubbed a lot.
If it were me, I would go with the BC stuff because ammonia fumes can do bad things to your lungs and the BC stuff is instantaneous.
Zonie