You might find something at a

You might find something at a gun store.

Blacken It

Right out of the advertising.

Blacken-It blackens brass, steel, nickel-silver, copper and most metals except aluminum and stainless steel. Will not chip or peel, conducts electricity and is fuel proof. 4 fl. oz.

If all else fails, follow instructions.

I have used it for brass wire.

Rich

Darkening Brass

You can get a commercial brass darkening solution (basically selenium dioxide solution) at some hardware stores - for example http://www.rockler.com/darkening-solution.

I've used this to darken a brass vanity sink (after a fire the restoration people had scrubbed the oil-rubbed finish off the sink thinking it was soot blackened!).  It does not go completely to black, but if you leave it long enough it does go to a very dark brown.

Wash and Seal

Hi IrishRover,

Be sure you wash the blackened items well or the chemical reaction will continue.

A clear overcoat would also be a good idea if the items will be handled.

Some of the chemical finishes react to fingerprints.

You might look into Jax Chemicals

http://www.sierrawestscalemodels.com/vanforum/discussion/311/blackening#Item_11

https://www.jaxchemical.com/jaxshop/shopexd.asp?id=61

Birchwood Casey

Brass Black is sold at any good gun shop.  Last 3-oz. bottle I got was $9.80.  Should last several years.  Follow the instructions on the bottle for best results.   For small parts, I just dip them into the bottle.  This stuff will blacken brass, bronze, and die-cast ("Zamac", "pot metal"), as well as iron and steel (except stainless).  Don't drink this stuff.