Bob, in the 80's on fancy fowlers,I did this brass black finish all the time. It requires a polished surface to look good. The best product is Brownell's 44-40.
Brass black does nothing to prevent rust. In fact, if you're doing a Hershall House antique job, you put brass black on the places you want to pit most before applying the Clorox.
That being said, I recently handled a French fowler I made in 1985 with such a barrel that was perfect. And the gun was well used. Just keep it clean and oiled.
Today I like a translucent brown finish.