Chris -
You can apply your finish using any of the good suggestions above, or anything else you can think of - but, that's only the beginning. You need a good base color to start with, of course. However, since pure black (or white, for that matter) seldom appears in nature, I always tone it down with a light overspray of "grimey black" or something similar. This also has the effect of making details easier to see.
For example, this Accucraft C-21 model came with a beautiful factory-applied black finish (except for the smokebox.) Here's what it looked like "out-of-the-box:"
Before
and After
I use an airbrush and water-based acrylic paint for this. Before the oversprayed paint completely dries, I go over the model with a damp Q-tip and clean off paint that covers anything I didn't want obscured - like lettering. Sometimes I don't clean it completely in order to leave it looking somewhat faded.
The next step is to wash the model with a black dye/water solution which settles into depressions and textures, like wood grain. The key to success here is to first "spritz" the entire model with "wet water" - water with a drop or two of dish washing detergent in it to break the surface tension, then use an eye dropper to apply the dye. If you let gravity be your friend, the resulting streaks should look natural. I usually lay it on pretty heavy. (I've always subscribed to the theory that anything worth doing is worth overdoing.) Once it (the model) is completely dry, I use my trusty dampened Q-tips again to remove or blend in excess dye.
Next, I dry-brush everything with a water-based acrylic PollyScale paints, right out of the bottle. Usually a dusty tan or light gray, and sometimes rust, but whatever, almost always a lighter color over a darker. I often use a flat, blade-type brush with somewhat stiff bristles to do this to make raised detail pop out and to sort of "half erase" some of the more extreme effects of the dye wash.
After dipping in the paint, I wipe most of it out with a paper towel and then brush even more out on to a pad of absorbent paper until I get just a tinge of color from the brush. I then kinda "whisk" the brush over the area I want to color - letting just the very tips of the bristles touch the work. Sometimes I have to make several passes over the same area to gradually build up the effect I'm looking for. It takes patience, but the results are usually worth it.
The hardest part of weathering is always the first step. Don't be afraid to mess up that beautiful pristine paint job. Almost everything I mentioned above can be undone or reversed. The black dye washes off easily and you can usually re-dry-brush with the original color to take it back to your starting place. It's easy to go overboard - so remember: subtlety is the key to realistic weathering. I've "taken a mulligan" many a time and found that subtle effect I was looking for after doing so.
After masking any glass surfaces, the final step is two or three liberal coats of Testor's Dullcoat from a rattle-can to seal the water-soluble dye and protect the finish. Once this is done, however, you usually can't go back and do any "unweathering" of your model.
Hope all this helps. Good luck and have fun with your project. Be sure to share pix of it with us when you're done and tell us what you've learned.