I'm assuming this is vegetable tanned leather and not chrome tanned?
It all depends on what the holster has on it - if it has been waxed - you might have a problem getting dye to take. You should be able to pick up a bottle of black dye from whee they sell shoe care products - polish, etc. - Meijers used to carry it - check around or call around. The only way you'll know is to try a small spot of the black dye in and area that can't be seen - such as some where on the back of the holster, etc.
One of the blackest dyes I use is "vinegaroon" - an old time dye that has been around for centuries. Made by disolving iron in vinegar. But, you'll probably end up with a "vinegar" smell for a while and usually, I leave mine sitting for several months before using to let the iron I've put in it do its job. Repeated applications will darken it. Many dyes are alcohol or water based which will also dry out the leather. After dying - you need to put natural oils back in to the leather. There are many things you can use - Mink Oil, various commercial leather rejuvenators, etc. When I get done making holsters/gun belts, etc. - I use pure virgin Olive Oil. It puts natural oils back in but will never turn rancid. It also makes a good lubricating oil - clockmakers have been using it for centuries.
A few years ago, I picked up several containers of "Marine Corps Black Dye" at Tandy Leather. It was available in various size containers. It's a very nice dark black - I use it primarily for dying burnished edges on belts, cartridge loops, holster seams, etc. Pretty easy to apply.
Again, though . . . whatever you get to use for dye . . . try it in a very small spot where it won't show to see if it will work or not. Good vegetable tanned leather should be used for belts, holsters, etc. - a lot of "cheap" gear is made from chrome tanned leather - two completely different tanning processes.