What is the equivalent to niello for brass?
I think you will have to age the alloy first, and clean the higher areas afterwards.

For tarnishing, you can use Palmolive, or any kind of washing liquid that has sulphates in it.
Another, more aggressive option, but only viable for solid, thick brass objects, is to use ammonia.
The most traditional choice, however, would be to use a mixture of salt and vinegar.
In all cases, you can leave the brass object in the liquid for an extended period of time, or rub it in with a piece of cloth, which will give you more control.
After the tarnishing, you can use either ketchup, a mixture of vinegar, salt and flour, lemon juice, or soap, to brighten the brass again.