You might be well advised to use a product, Feibing's #12 Deglazer, available at Leather Factory/Tandy or any retailer who stocks Feibing's products. Use it to strip any finish on the holster completely. Make sure the holster is completely dry and then apply a LIGHT coat of Pure Neatsfoot Oil, and then immediately apply the black dye. The oil will help carry the dye into the fibers. An alternative method would be to strip, dry, and then wet the holster thoroughly with plain water and then immediately apply the dye. This method could have a detromental effect to the wet molding of the holster and you might have to remold it. In either case you should probably apply at least two or three coats of dye with thorough drying and buffing between coats. When the whole thing is finished I would recommend you apply Feibing's TanKote and a sealer. This is just the way I would approach it, others may have a different preference.
Paul