Brass turns "black" when cleaning due to over-use and misuse of polish. The biggest challenge to upkeep most metals, including brass is the removal and inhibition of tarnish. All substances, especially metals, oxidize when exposed to air. Once tarnish is removed, a chemical barrier should be created between the bare metal and the air to inhibit the process from re-occurring.
Many people over-use and flood metal surfaces with polishes believing that they are better protecting the surface. The more polish, the more protection. Wrong assumption! More polish creates a smudging problem since fingerprints and human body oils "dissolve" the solvency of the metal polish. Additionally too much polish may discolour the surface. Only a trace amount creating a THIN film should be applied. Therefore, an adequate amount of metal polish should be applied and spread out an amount on an absorbent rag. Then, let the rag dry out for a minimum of 24 hours before placement onto most metals. Apply this trace amount of polish with the grain of the brass with one hand while buffing it out in a rapid motion (creating friction) with the other hand. This burnishing action will harden the polish (like "spit shining" a shoe) and create a surface far more difficult to smudge or discolour.
Dealing with "raw" brass instead of finished lacquered brass! The reaction between raw metal and chemicals can create the condition. The care of most metals is a 2 step process:
Cleaning: (for light soils) the use of isopropyl (rubbing alcohol) applied with the sponge side of a light-duty, "white-padded" scrubbing sponge with the grain of the door. In the event of tougher scuff marks, flip over sponge and gently agitate with the grain of the metal with the white scrub pad (for heavier soils). Dampen sponge side with water, and apply a light scouring low abrasion crème onto it. Work product into sponge, and then stroke it onto your door with the grain. Once completed, wipe surface thoroughly clean with a clean, soft rag. Once surface is cleaned, then go to the next step.
Polishing: One of the best tools which provide just the right amount of oil onto metal is a "yellow" treated dust cloth. Wipe down brass with this cloth and then buff it dry with a soft, cotton cloth. This trace amount of oil in the cloth should not smear or discolour, especially after buffing.
Lacquering can be done at home, but all old lacquer must be removed first, and the surface completely cleaned (no fingerprints or cleaner on it) before spraying the lacquer on evenly in multiple thin coats. It is hard to do well. Keep decorative items dusted and clean. Wash in sudsy, lukewarm water, rinse and dry. Never use hot water on lacquered items as it loosens the lacquer; do not polish them or soak them in water.
Olive Oil. Brass will look brighter and require less polishing if rubbed with a cloth moistened with olive oil after each polishing. Olive oil retards tarnish.
Unlacquered brass tarnishes when exposed to air. A weekly wiping with a little liquid ammonia on a soft cloth will help keep unlacquered brass shiny. Use a commercial cleaner (available in hardware stores) to remove tarnish. On antique brass, test the cleaning product to be sure of obtaining the desired effect. Some methods not only clean tarnish but also remove the mellow colouring of age that is desirable on old drawer pulls and other accessories.
To polish antique brass pieces: wash in hot, soapy water to remove grime, wax, etc. Rinse and dry. Moisten a soft cloth with boiled linseed oil and rub on the brass surface until all the dirt and grease have been removed. Polish with a soft cloth. To polish for a soft finish: wash in hot, soapy water, rinse and dry.
Apply polish with a soft cloth and rub to remove tarnish. Wipe off excess paste and polish with a clean cloth. To remove heavy tarnish, difficult stains and corrosion: wash in hot, soapy water or a weak ammonia and water solution and rinse. Dampen a soft cloth in hot vinegar, then dip in table salt and rub the brass, or make a paste of flour, salt and vinegar. You may need several applications. When the item is clean, wash in hot, soapy water, rinse and dry thoroughly, then polish with a cloth moistened with lemon oil. If preferred, dip a slice of fresh lemon into table salt and rub over the corroded area. Wash, rinse and dry carefully.
This was lifted from an American Hardware FAQ page and altered slightly to suit our applications.
Thanks to Michael Wolf for finding it.