Gold doesn't accept these chemicals at room temperature.The simplist
solution might be to heat the pieces with your torch , without the
boric acid/alcohol solution, until they blacken, then gently polish
the raised portions......don't pickle the pieces, or you will have
to start over. You could also mask all surfaces you wish to keep
yellow with nail polish, and electroplate the pieces with rhodium at
such a high voltage that you "burn" the rhodium in the process...it
will blacken very nicely, and simply will not come off the recesses.
Anyone who has done rhodium plating has experienced this phenomenon
if using somewhat depleted solution to plate. If you don't have the
rhodium in your shop, phone around to the trade shops in your
area...almost all of them will use it.
There are manufacturers who plate with ruthenium, but I don't know
of any in North America. This is a very black, extremely durable
finish. Probably not an option, but the rhodium would work if you
don't find the heat oxidation porduces a satisfactory colour.