There are a few tips on these threads:
The black spots are typically where the lacquer or clear coating is starting to wear and pit. This can happen on both solid and brass-plated pieces. So now you have a choice: remove the lacquer entirely or just clean with Brasso or Bartenders Friend- that should remove the stains for awhile but they will come back. Removing the lacquer is a tedious process, especially for a large piece, but it can be done. But then you either have to re-coat it or leave it as a burnished brass piece.