The stain that you speak of is an oxide that results when high
copper bearing alloys-- copper, bronze, brass, tumbago-- are
vulcanized. ) Sulfur in the rubbers reacts with the copper, I think.)
You can certainly use bronze masters (after all, the "button
formers" are bronze).
I sometimes do, and I find that the oxide's ill effects eventually
settle down, especially after the mold has been "pounced" (dusted)
with talc or cornstarch a few times and shot with several waxes..
(Remember that talc is not good to inhale....)
What I often do when using bronze masters is silver plate them.
Just a flash plate will do. This solves the problem pretty much
entirely. One concern here: make sure the master is quite clean prior
to plating. If not, the silver plate will flake off the model,
remain attached to the inner mold surface and -- at the most
inopportune time-- come loose and find its way into the wax. This
doesn't happen often but you can't discount Murphy's Law.
Take care and hope this helps.