I have a bright chestnut and am thinking about getting her brown tack. Since she is just starting under saddle she is going to be changing a lot and I'm looking at cheaper saddles. Brown seems to be cheaper than black anyway so thats a score, but I'm finding that a lot of the brown saddles seem to almost be black after a lot of use.
We talk a lot about how to darken leather and I'm really good at that, but is there any way to "bleach" a saddle to get it closer to its original color?
Not in a way that is good for the leather, no. It is the nature of leather to darken and develop a patina.
The only real way to do it is to have the leather dyed with a pigment dye vs aniline. Basically you are "painting" the leather so you will have a finish like on automotive leather. Unfortunately, this also "seals" the leather so conditioners can't soak in. The leather will stay light until you wear through the dye. Of course, by the time that happens your tack will also probably be dry-rotted.
Buy something that was lighter in color to begin with and it won't get as dark as quickly. That awful london tan color was popular for a while, so you might be able to find something that started out really light.
ETA: once you find something that is a good color, only use non-darkening conditioners. Hydrophane makes one that is nice. The downside is that they don't condition as much as good ol' neatsfoot oil.