A few years ago I picked up a product from Caswell Plating that appeared to give a true black color but have yet to try it.
Thanks again. The stuff I've been using lately is the Super Blue. I have some PermaBlue on hand too.
Funny about your Caswell Plating kit: I have an Eastwood Automotive blackening kit sitting on a shelf. The instructions say to dilute the blackening stuff. It makes a large amount and comes with a big container so you can submerge your parts in the liquid. So far I haven't been inclined to play chemist when the little bottles of Birchwood Casey stuff will do.
Interesting point by Gregr. If the wheels are hard to get off, I would think they might also be hard to press back on.
Would postwar wheels even fit?
I'm with Norton, clean them in place.