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I think the answer's pretty straightforward:

Blue Wonderâ„¢ Gun Blue will only bond to ferrous metals and will not bond to stainless steel, aluminum or case hardened steel.

from http://www.bluewonder.us/TechnicalSupport.html

BLUING: Note: It will only work on carbon steel, not any metals that won't rust, like stainless.

from http://www.shootersolutions.com/instvig.html

Here's a typical do-it-yourself bluing solution:

Copper sulphate 1/2 oz.
Iron Chloride 1 lb.
Hydrochloric acid 4 oz.
Nitric acid 1/2 oz.
Water 1 gallon

Then allow to dry for several hours; place in above solution again for 15 minutes; remove and dry for 10 hours. Place in boiling water for 1/2 hour; dry and scratch brush very lightly. Oil with mineral or linseed oil and wipe dry.

I sure as heck wouldn't want to place my tuba in that kind of stuff--and it's clear from the makeup that it will work only on carbon steel.

So, sure, go ahead and blue that carbon steel tuba!

(BTW, it took me no more than 2 minutes to find all of this out with a simple web search).

It is possible to electroplate iron over non-ferrous metals. My soldering iron uses iron-plated copper tips, for example.

You're going to have a tough time finding commercial platers who will do it, because there's not much demand. But I'd give it a try using a ferrous chloride bath and an iron anode. Start off with scrap brass to figure plating current and solution concentration.

You should be able to get at least a cosmetic coating of iron without much trouble.