Factory transformers (wpc) were not painted black, they were blued (just like the ramp flaps). That is the reason why the metal appears to be black, but when you see it under a white light it becomes blue-ish.
There are two methods to blue an iron-containing metal: hot and cold.
Hot is more permanent but you can’t do this to your transformer because hot blueing involves dipping the metal in liquid chemicals.
Cold method is what I use to blue the outside of the transformer. You buy the blue chemical from gun stores mostly (it’s an acid) and follow instructions on the box. It’s very important to do the method right and apply a very thin layer of machine oil once a year or so. This keeps the metal from rusting because cold blue doesn’t protect as much as hot blue. You also keep your machine away from moisture (duh... common sense)

PS: good call on the wire brushing! I do this too on rusted metals I can’t glass-beat with excellent results!