First of all, unless you are experienced at this sort of thing, it's not a really good idea.
If the belt has already been dyed, removing all the color will be next to impossible. There will always be some residue of the color in the nooks and crannys. The dye will not come off evenly, because it absorbed at different rates in the leather. Use the same Deglazer mentioned in the next paragraph.
If pre dye is not a factor, you can remove most (not all) of the top finishes, if any, with a product by Feibing's "#12 Deglazer Cleaner" available at Tandy/Leather factory or similar store carrying Feibing's products. Ask for instructions where you buy it, because there are no instructions on the can.
Chances of matching the new holster to the belt will be about the same as winning the Mega Lottery with one ticket, unless when finished you dye them both black.
The only sure way to get a match is to remake the belt out of the same side of leather the holster is made from and dye them both at the same time with the same dye, then your chances will be about 50/50.
An alternative is to buy pre dyed leather from Wickett & Craig and make both items from the same side of leather and at the very least your color will be constant.
By all means do take it to Tandy and ask for advice, (good idea) but be aware that regardless of what they tell you, the odds of success are going to remain as I mentioned above. Good Luck
Edited to add: Tooling the finished belt will be very difficult, (impossible if it still has the finishes on it). It will still be very difficult to get a good job tooling after deglazing because the leather won't case (moisten) properly after the deglazer is used.
Paul
