I’ve had excellent results from Black Max brand from Rio Grande,
but you do have to let the patina dry overnight for best
adhesion. Trying to speed it along with too much heat seems to
make it dry funny, get patchy or flake. If you want an extreme
black, try opaque black laquer, usually sold as a jewelry
oxidizing laquer. This holds up better to buffing and is
unaffected by chemical cleaning solutions.