Good points all ... I do know that the cores are made from low carbon steel, and it stands to reason the bells are also low carbon or they wouldn't shape the way they do without cracking. Still ... wouldn't hurt to experiment a bit on the bottoms of the transformers. I'd hope for a two tone effect with the bells going darker than the cores. The Van's Gun Blue is also more of a metal dye that the other cold bluing solutions, so maybe the carbon isn't the issue you'd think it'd be. You can also build layers for a darker color.
Then again, I'm kinda diggin' on the naked look right now ...
They cleaned up real nice with some scraping and elbow grease. The secret weapons for getting the glue off the plates was a flat scraper and ... tada ... drywall screen? 3M polishing pads in various grits and a good scrubbing with a Never Dull polishing pad gave a nice smooth sheen to the bare metal. I use the Never Dull on the bike for all the chrome (including hot exhaust), so I know it holds up to a lot more weather than the amp should ever see - even if I park it under a picnic table??
It leaves a protective seal on the metal so rust isn't an issue.
Project's on hold right now due to spring planting, so I got time to ponder while I'm knee deep in the soil ... the soil, the soil, the soil!