Percussion plains or plain rifles were 1830 or after, and in some areas 1840 or after, as percussion caps were not commonly available until that time. Hanson considered the era of the plains rifle from about 1830 to 1865. The definition of the plains rifle or "mountain rifle" is also generally considered to be those rifles made in St. Louis.
Some of the makers browned their rifles, and some blued them, so to be exact, you should consider who's rifles that you are trying to duplicate, and duplicate their ways. Otherwise, since the rifles in general were both blued and browned, you could make your rifle however it appeals to you.
Most of the plains or mountain rifles were iron trimmed. What kit are you doing? Brass would probably go better with blued barrel.