Hi Johni, Franco
In my opinion, Johni, your tsuba is a true shakudo. Based on a few dozen analyses I've done of similar pieces I'd say it contains between 0.5 to 1% gold, probably a similar amount of silver and around 0.5 to 0.8% arsenic. I would be very surprised if those copper patina patches didn't come back to a good black, however, they won't while that copper patina is protecting what's underneath. It may be that copper seppa caused the rubbing and there is now more copper on the surface then when it was new.
The arsenic is almost certainly a deliberate addition, it's part of a pre-alloy that was made prior to the addition of the gold and silver. This procedure is described in older texts.
I've now amassed a few dozen analyses of shakudo pieces, Ko-mino, Ko-goto, and the rest, right through to late 19th cent. I can't see any real patterns in terms of age though so calling something Muromachi shakudo is pretty much a spit in the dark in terms of accuracy, imo.
The depth of the black patina is not inevitably linked to the gold content. So, a supposed link between 'quality' in terms of patina richness and 'quality' in terms of imagined gold content can't convincingly be made. I have images and analyses of samples with as little as 0.5% gold and one with as much as 7%, It's almost impossible to tell the patina colours apart.
I have yet to encounter an antique (pre-1900) shakudo that was only copper and gold. Silver is always present and in probably over 95% of samples there is also a trace of arsenic. The arsenic is important and, I believe, plays an important role in those shakudo that do exhibit a tendency to 'self heal'.
The colour of shakudo patina is very much effected by the surface finish of the alloy. Older pieces often have a subtle micro-corroded surface. I believe that this results in the appearance of a deeper black patina as the light it reflects is somewhat diffused whereas a finer mirror finish reflects the light more directly resulting something of a bluish tint.
There are many other factors that can be considered when trying to asses old shakudo. Under certain conditions rubbed shakudo can reform more of a copper patina, perhaps a different sort of body chemistry in the last handler of the piece is the key.
All this to say i think we ought to be a lot more cautious and less confidant when judging these things.