If the shadowing looks dull or slightly purple it is fire scale.
Unfortunately it will have to be abraded off by heavy polishing
with tripoli. If the shadow is in a confined spot where you cannot
get to it with a large buffing wheel you might want to use a small
buffing wheel and tripoli or zam on your Foredom. It will take time.
The best bet is to not get fire scale when you solder. There are
several fluxes that can be used to cover the entire piece before
each solder step. They will prevent fire scale when used properly.
There is also Prips. The formula can be found in orchid archives.