Welcome to Orchid!
We also use Silverblack in our studio, as it stores well, and gets an
extremely black surface. It can also be diluted with water to get
shades of gray. A rough surface on your silver will hold the
oxidation better than a polished surface, and makes for a more
consistant dark color. What we do is put the item to be oxidized on
a warming tray, to get it hot first. While on the tray, we paint on
the oxidizer with a paint brush. Do this before you do your
polishing, as polishing compounds can get greasy residue in the
recesses. The heated metal will help the chemical reaction between
the metal and the oxidizer, and help make the surface blacker. After
polishing, I’d avoid putting the piece in the ultrasonic cleaner, as
that tends to strip out the oxidation.