I've done cold black oxide (same as Caswell), hot black oxide (outsourced), parkerized, cerakote (outsourced), and powdercoat (outsourced). The cold black oxide looks pretty good and is very easy. What I've found is this; the appearance/color-tone of the cold black ox and parkerized finishes are entirely dependent on steel composition, hardness, and surface finish. The hot black ox, on the other hand, is a nice consistent color tone.
