I use cold (gun metal or tool steel) blueing you can get it from enco tools in a quart size. or at big 5 in little bottles 5oz or so.
to get the black coloring you have to put a good smooth, sanded finish on the steel, for instence if you have a bead blasted finish, it comes out more brown.
hope that helps
matt

 Depending on what the piece is like, large, small, rough, smooth  flat or with many surfaces... clean it via chemical, wire brushing, sandblasting or whatever means you have... then give it a thin spray coat of motor oil or wax or  ... etc. and bake it in an oven or kiln or heat flame it in a metal box so it comes up to a fairly consistent temperature.  It will bake the oil on to become a great black.  It is a system I generally prefer but also because I have the kiln and facilities.  bpfink

I have had a lot of luck with a product called E-tec 521-B from
EPI. It's a black wax emulsion so it needs to be used for an interior application. It's the blackest finish that's no toxic. You can brush it or spray it on. I've used it for years. Good stuff.
http://www.epi.com/inhibitors-corrosion/resin-and-wax.html#521

Loved that eagle by the way. Nice job. It's rare to see something done with found objects that is surprising. Well done.