You might try Griffiths silver black.
I'm very fond of this stuff, and I use it all the time. It's very
easy to use, requires no heating, keeps indefinitely, and provides an
even, durable black finish. A little bit goes a long way, and it can
be brushed on or used as a "dip" for production pieces.
Just take care when handling it - it is hydrochloric acid based. It
won't generally hurt you, but if you should ever chance to get it in
a teeny papercut you didn't know you had, it stings like all get out.