I tumble in Ceramic media (home built tumbler, Harbor Frt media), then wire brush to get a clean piece. Then heat the item till hot to the touch and apply wax. Or I leave it cool and apply Rustoleum's Matte Clear finish. The rust protection in Rustoleums products add a touch of blackening to low spots and leave highlights with that "Steely" look. To blacken your pieces, heat to about 500ºF then quench in motor oil, a nice black finish will result.
Matt