If you boil a rifle that was browned will it convert to black oxide (bluing)

Yes; that is exactly what rust bluing is; you convert ferrous oxide (red) to ferric oxide (black) by boiling it; or you can steam it.
However...depending on the rust, you might get a matte finish. Very matte.
It will also be saturated with oil.

Let's buld on that oil part.. if not degreased the finish will be splotchy

Though birchwood-casey plum brown will turn a nice dark blue when boiled.. very matte