I've had good luck with Brownell's Oxpho Blue. If you can bead-blast the mag shells, Oxpho Blue will give you a nice matte finish.
After de-greasing, heating the metal (I use a propane torch to heat the steel until water sizzles on the surface - not much hotter than that) and using de-greased steel wool to apply the blue both seem to help at achieving an even finish.
For a darker finish, repeat the process. When you're done, rinse with HOT water, dry thoroughly and apply your favorite preservative.
I think of my gun as a tool for converting worthless human garbage into valuable fertilizer.