MIMF

I need to darken some brass nails.
Anyone know what might work for this? (besides lots of time time)

I would try vinegar.

Brass Darkening Solution from Rockler or brown or green patina made by Jax. You can search online for ways to do it with liquids you may already have at home. It just takes longer that way.

Birchwood Casey makes a product for brass. You can probably find it discounted somewhere.

http://www.birchwoodcasey.com/Finishing ... 0d6728fd8f

Sulfur containing compounds will generally tarnish copper bearing metals. Water that eggs were boiled in come to mind. Also, there is a substance called "liver of sulfur" that is often used to antique copper, which is the main ingredient in brass.

You can mix up your own solution of brass black using cupric carbonate (available from ceramics supply places -Georgies has it) and household ammonia (apprx 1 tsp per pint). It's a tricky process if you want consistent results. Everything needs to be cleaner than clean. The solution needs to be warmed up to about 140º or so work most effectively. The ammonia evaporates out of solution and stinks up your workspace if you don't have good ventilation.

I'd second the Birchwood Casey solution. Just daub it on with a q-tip. Amazon had it in several sizes but most sporting goods stores ought to stock it as well.
The Birchwood Casey MSDS for brass black includes: Selenious Acid; Cupric Sulfate; Phosphoric Acid. These are corrosive chemicals and shouldn't be dumped down the sink.