I was just reminded of an old gunstock stain I've used on knife hilts and stocks,

and thought I'd pass it along.

Note: it uses acid, so you have to be careful making and using it. 

Here goes:
1 part acid(nitric or muriatic work, the original formula specified nitric)
5 parts water(can use more, but the acid has to be strong enough still to dissolve the steel wool)
Steel wool(back then it was iron filings)
Mix the liquids in a glass jar by slowly adding the acid to the water, make sure it's thoroughly mixed.  Then add a little steel wool. It'll react, you'll get some foaming, let that die down, then add a bit more. Keep going until the solution is saturated and won't dissolve any more, then loosely cap and set somewhere cool for a couple of days.

To use it, you make sure all previous finish is removed from the wood, then you'll need a heat source, a heat gun will work or a gas burner, and linseed oil. And use gloves.

Use a swab of some kind to wipe the finish evenly over the wood, keep doing it for a minute or two as you need it to soak into the wood. When you're sure it's soaked into the wood evenly, wipe off the excess.

And now you work it over the heat, or the heat gun over it. What happens is the solution carries the dissolved iron into the wood, and the heat makes it rust and darken the wood.  It has to get hot enough to do this, so leather or some other type of heat-resistant glove is needed.  Keep working the heat evenly over the surface until it's a bit lighter than you want, and then stop the heat and hit the wood with linseed. Not just a little, wipe or brush it on HEAVY; it kills the traces of acid left and stops the process, but it will darken a bit more before the action stops completely. Keep it wet with linseed for at least 15 minutes, then let the excess drip off for a while, then wipe it off and give it- depending on weather conditions- at least two days and probably a week to dry completely. If in doubt, give it more time to make sure it's dry.  After that you can use whatever surface finish you want on it.

This works very well to darken wood and bring out the grain.  You can darken it just a little, or make it just about black.  But if it gets darker than you wanted, the only way to change that is to sand it down and do it all over again, so I'd suggest experimenting on some scrap pieces before you put it on that stock or grips you want to use it on.