You can make a super-saturated solution of copper carbonate (CuCO3Cu(OH)2) and regular, over-the-counter ammonia, The copper carbonate could be purchased at a good chemists. these days.... It is also used in pottery to colour glazes with so that might be a source.
Mix it outdoors as the ammonia is not to good for you and stinks like hell. Add the copper into some ammonia and stir well keep adding until the liquid can no longer dissolve the crystals.
Keep in a tight closing plastic bottle of tub. To use, degrease the brass and place in a shallow dish with some of the solution. It helps to turn the part and stir up in the liquid once is has the darkness you want, remove and rinse well in clean water. The solution can be reused quite a bit so pour it back in a container.
As a side note if you need to bend small wooden parts, (like planking for a model ship) if you soak the strips of wood in ammonia it softens the long strand cells of the wood so much that you can tie a knot in it! Rinse under running water once 'soft' enough and bend around to the desired shape. leave to dry. then glue in place.